Transliteration for Low-Resource Code-Switching Texts: Building an Automatic Cyrillic-to-Latin Converter for Tatar

Chihiro Taguchi∗, Yusuke Sakai∗, and Taro Watanabe {taguchi.chihiro.td0, sakai.yusuke.sr9, taro}@is.naist.jp Nara Institute of Science and Technology

Abstract mixes Russian words, it is not easy to obtain pure Tatar dataset for developing a language detector. We introduce a Cyrillic-to-Latin transliterator for the based on subword-level Existing methods are based on either Tatar mono- language identification. The transliteration is a lingual rules or a huge bundle of ad-hoc rules aimed challenging task due to the following two rea- to cover Russian-origin words (Bradley, 2014; Ko- sons. First, because modern Tatar texts often rbanov, ..). The experimental results in Section contain intra-word code-switching to Russian, 6 demonstrate that the former monolingual rule- a different transliteration set of rules needs to based transliterators show low accuracy because applied to each morpheme depending on the Russian words are not supported. The latter exten- language, which necessitates morpheme-level sively rule-based transliterator has better accuracy, language identification. Second, the fact that Tatar is a low-resource language, with most of but still misses a certain amount of words. This the texts in Cyrillic, makes it difficult to pre- implies that a strictly rule-based method requires pare a sufficient dataset. Given this situation, an ever-lasting process of adding rules ad hoc for we proposed a transliteration method based exceptional words to further improve the accuracy. on subword-level language identification. We This is obviously unrealistic and inefficient. trained a language classifier with monolingual In this study, in contrast, we pursue a simple Tatar and Russian texts, and applied different yet high-accuracy automatic transliteration system transliteration rules in accord with the identi- fied language. The results demonstrate that our from Cyrillic Tatar to Latin Tatar. We prepare two proposed method outscores other Tatar translit- sets of simple rule-based monolingual translitera- eration tools, and imply that it correctly tran- tion rules for Tatar and Russian each. In addition, scribes Russian loanwords to some extent. we train a classifier that automatically determines Tatar or Russian for each subword. Each token 1 Introduction is then transliterated to Latin Tatar by applying Modern Tatar has two orthographies: Cyrillic and the rules of the detected language to its subwords. Latin. The two alphabets are mostly mutually com- The results demonstrate that our proposed method patible when an input string consists of only Tatar- achieves higher accuracy than the previous tools. origin words. Effectively, however, modern Tatar Also, our proposed method demonstrates higher ac- has a massive amount of Russian loanwords, and, curacy than the transliterator with only Tatar-based in colloquial texts, even a whole phrase may be rules, indicating that the method can correctly pre- switched to Russian. This linguistic phenomenon dict and transcribe Russian words to some extent. is known as code-switching or code-mixing. A difficulty of transliteration from Cyrillic to 2 Tatar: Linguistic Background Latin lies in the following two facts. First, a dif- Tatar (ISO 639-1 Code: tt) is a Kipchak language ferent set of transliteration rules has to be applied of the Turkic language family mainly spoken in to Tatar and Russian words. This requires a lan- the Republic of , . The number of guage detection for each token, or worse, for each the speakers is estimated to be around five million morpheme, which would additionally require mor- (Eberhard et al., 2021). The canonical word order phological analysis. It is expected that a full imple- is SOV, but allows for free word order with scram- mentation of such a system will produce heavy pro- bling. Morphological inflections and declensions cesses. Second, because modern Tatar frequently are derived by means of suffixation. The suffixa- ∗Equal contribution tion obeys the harmony rule where backness 133

Proceedings of the Fifth Workshop on Computational Approaches to Linguistic Code-Switching, pages 133–140 June 11, 2021. ©2021 Association for Computational Linguistics https://doi.org/10.26615/978-954-452-056-4_018 (or frontness) of is kept consistent. locative suffix -ta (-ta “at, in”) is attached in the Through the long history of language contact example, as Russian loanwords may take Tatar suf- with Russian, modern Tatar contains a large amount fixes, causing within a token (intra-word CS). of Russian loanwords. Most of the speakers are (1) Bezneң klassta kyzlar sigezenqedәn bilingual along with Russian, and younger genera- qә baxlagan ide. tions living in urbanized cities tend to have better translit.: Bezneñ klassta qızlar sigezençedän competence in Russian than in Tatar. This bilin- eçä ba¸slagan˘ ide. gualism leads to frequent code-switching (CS) in “In our class, girls used to start drinking by the text and speech, particularly of colloquial genre eighth grade.” (Izmailova et al., 2018). 3 Related Work 2.1 Tatar Orthographies The mainstream orthography for modern Tatar is Code-Switching. Even though CS has attracted the Cyrillic alphabet, which comprises the Russian researchers in NLP, the lack of resource has been standard alphabet and six extended Cyrillic letters. a major difficulty, because CS is an exclusively Cyrillic Tatar is mostly used by Tatars living in colloquial linguistic phenomenon and CS texts are Russian-speaking countries, while the Latin alpha- seldom recorded. Jose et al.(2020) enumerates bet is used by Tatars living in other areas such as a list of available CS datasets at the time of the Turkey and Finland. publication. In terms of both the availability of Until 1928, Tatar was exclusively written in the datasets and the popularity of research, CS lan- . Along with the latinization project guage pairs in trend are Hindi–English (Srivastava prevailing in the in 1920–30s, a Latin et al., 2020, Singh and Lefever, 2020), Spanish– alphabet system yañalif was introduced to Tatar. In English (Alvarez-Mellado, 2020, Claeser et al., 1939, for political reasons, yañalif was superseded 2018), Arabic varieties and Modern Standard Ara- by the Cyrillic alphabet which has been officially bic (Hamed et al., 2019). used in Tatarstan as of now. After the demise of As for the studies of intra-word CS in other lan- the Soviet Union, with the resurgence of the move- guages, Mager et al.(2019) for German–Turkish ment for restoring the Latin orthography, a new and Spanish–Wixarika, Nguyen and Cornips(2016) Latin-based orthography system was adopted by for Dutch–Limburgish, and Yirmibe¸soglu˘ and Ery- a republic law in 1999 (National Council of the igit˘ (2018) for Turkish–English have a similar ap- Republic of Tatarstan, 1999). However, the law proach to ours. The differences from ours are that soon lost its validity in 2002 when a new para- Mager et al.(2019) employs segRNN (Lu et al., graph stipulating that all the ethnic languages in 2016) for segmentation and language identification, Russia must be written in Cyrillic was added to and that Nguyen and Cornips(2016) uses Morfes- the federal law (Yeltsin, 2020). The current Latin sor (Creutz and Lagus, 2006) for morphological alphabet (2013Latin henceforth) based on the Com- segmentation. However, our task that combines mon Turkic Alphabet was officially adopted by a language detection of intra-word CS and translit- republic law in 2013, and is commonly used in eration has never been undertaken in any of these Tatar diaspora communities (National Council of studies. the Republic of Tatarstan, 2013). We define that the term “Latin alphabet” used Tatar Transliteration. At the time of this writing, in this paper refers to 2013Latin. A detailed the following tools are available for Tatar Cyrillic- rules for the conversion to 2013Latin is given in Latin conversion. The Tatar Transcription Tool Timerkhanov and Safiullina(2019). (TTT henceforth) (Bradley, 2014) is a translitera- tor published online by Universität Wien as a part 2.2 Code-switching in Tatar of the Mari Web Project. speak.tatar1 is an anony- An example of the former is displayed in (1) with mously developed transliteration service. FinTat2 is transliteration in the Latin alphabet and the trans- a transliteration tool developed as a part of the Cor- lation. Underlined klass (klass “class, grade”) is pus of Written Tatar (Saykhunov et al., 2019). Ay- a Russian word naturalized in Tatar, though it is landirow is a strictly rule-based transliteration tool pronounced with Russian phonology and therefore 1https://speak.tatar/en/language/converter/tat/cyrillic/latin requires a different transliteration. In addition, a 2http://www.corpus.tatar/fintat 134 available online that extensively covers Russian- form high accuracy8: the number of dimensions is origin words as well as Tatar-origin (Korbanov, 16, the minimum and maximum character n-gram n.d.). The transliteration system employed in Fin- sizes are 2 and 4. Hierarchical softmax is used as Tat is based on the Latin alphabet used by Tatars in the loss function. Together with this representa- Finland, whose orthography is somewhat different tion, the subword embeddings are annotated with a from 2013Latin. language label (Joulin et al., 2017). It is worth noting that, unlike Mager et al.(2019), 4 Method we do not employ deep learning approach. While the task in Mager et al.(2019) is multi-labeling, We transliterate Cyrillic Tatar to Latin Tatar word our language identification task is a binary classifi- by word, as each word does not affect other words cation with a low-resource training dataset; for this in Tatar transliteration3. reason, deep learning is too superfluous and heavy Taking into account the fact that Tatar has intra- for achieving our task. word CS, we created a classifier that detects a lan- To evaluate the performance of our model, we guage (Tatar or Russian) for each subword. To apply BPE also to the test data in the same manner, implement the language classifier, we prepared two and predict a language label for each subword. Ad- monolingual corpora of Tatar and Russian. Given jacent subwords are, when possible, combined to the lack of pure Tatar texts without CS in modern form a longer subword with a single language label texts4, we employed Tatar translation of Qur’an5 for the sake of better accuracy; that is, for example, (19,691 words with duplication) translated in 1912 when two consecutive subwords are both labeled that contains no Russian loanwords in order to as tt, then they are combined into one subword la- avoid noise to train the classifier. Its Russian coun- beled as tt. Finally, each subword is converted to terpart6 (21,256 words with duplication) was trans- the Latin alphabet with the transliteration rules of lated by the Ministry of Awqaf, Egypt. the predicted language, and combined into a word The training process is as follows. First, the as an output. words collected from the dataset were automati- cally divided into subwords by the Byte Pair - 5 Experimental Setup coding algorithm (Sennrich et al., 2016). Baner- jee and Bhattacharyya(2018) reports that, unlike For the evaluation of the performance, we prepared Morfessor, BPE can flexibly solve the OOV prob- 700 sentences (shuffled; 8,466 words with duplica- lem because some subwords are character-level tion, 5,261 without duplication) from the Corpus segments. In our case, due to the meagerness of of Written Tatar (Saykhunov et al., 2019) and their the monolingual training data, we employed BPE Latin counterpart as the gold data that was man- to avoid the OOV problem. Then, assuming that ually transcribed by us and verified by a native longer subwords are less ambiguous with respect to speaker. Also, we annotated the Cyrillic text data labels to be assigned, we took the longest match to so that CS Russian morphemes are tagged. Accord- make it easier to distinguish between Russian and ing to this, the text data contains 1,009 words (with Tatar; for this reason, the subword merge opera- duplication) with a Russian morpheme, and 598 tion was repeated until no further merge was possi- words (with duplication) with intra-word CS. ble. The obtained subwords are then represented in For evaluation metrics, we calculated BLEU and subword embeddings using fastText7 (Bojanowski longest common sequence (LCS) -measure for et al., 2017). The classification model is the su- each letter in a word as well as word accuracy pervised classifier provided by fastText, with the (ACC) and character error rate (CER). The calcula- following hyperparameters that are known to per- tion of LCS F-measure and ACC is based on Chen et al.(2018). 3The code is available here: https://github.com/naist- nlp/tatar_transliteration. A demonstration page is published To compare the performances of Tatar monolin- on the website: https://yusuke1997.com/tatar. gual transliteration and of Tatar–Russian bilingual 4 In the evaluation dataset we prepared for this study, 1,009 transliteration (.., our proposed method; “tt-ru words out of 8,466 contained at least one Russian morpheme. 5https://cdn.jsdelivr.net/gh/fawazahmed0/quran- hybrid” henceforth), we also evaluated the data api@1/editions/tat-yakubibnnugman.json with solely Tatar monolingual transliteration rules 6https://cdn.jsdelivr.net/gh/fawazahmed0/quran- api@1/editions/rus-ministryofawqaf.json 8The training setting was inspired by the blog post by 7https://github.com/facebookresearch/fastText fastText: https://fasttext.cc/blog/2017/10/02/blog-post.html 135 BLEU LCS F-score CER ACC # correct sentence # error word speak.tatar 0.869 0.953 0.049 0.952 67 1,747 TTT 0.879 0.956 0.054 0.946 121 1,505 Aylandirow 0.971 0.994 0.009 0.991 362 526 tt-based 0.968 0.989 0.011 0.989 365 552 tt-ru hybrid 0.981 0.994 0.007 0.993 437 332

Table 1: The experimental results with 700 sentences (5,261 words without duplication). CER and ACC are complementary in probability (i.e., ACC = 1 − CER). Note that the transliteration result is uniquely determined as the proposed method is rule-based.

(“tt-based” henceforth). For the comparison with monolingual transliterators. However, it needs to the existing transliteration tools, we computed the be examined that tt-based also gained higher scores scores of speak.tatar, TTT, and Aylandirow9. than the other monolingual ones, even though it does not support Russian loanwords. This is due 6 Results to the fact that their transliteration rule sometimes As shown in Table1, the experimental results does not follow the rules of 2013Latin. For exam- demonstrate that our tt-ru hybrid marked the best ple, tәnky~ “criticism” is transliterated as tän- score in all the metrics. CER is the lowest, mean- qit’ (with a hamza at the end) by the TTT, where ing that the total number of mistakes at a charac- it should be transliterated as tänqit according to ter level is the fewest. The difference is evident 2013Latin. In fact, due to the -facto absence between monolingual transliterators (in particular, of any institution that regulates the Latin orthog- speak.tatar and TTT) that do not support Russian raphy, different varieties in spelling styles can be loanwords and bilingual transliterators (Aylandirow observed on the Internet. For this reason, the seem- and tt-ru hybrid). Between the two groups, there ingly high scores in tt-based are merely a product is more or less a difference by 0.1 points in their of the orthographical consistency. BLEU scores. Furthermore, our monolingual tt- Keeping this in mind, the improvement in the based marked higher scores than the other two scores between tt-based and tt-ru hybrid indicates monolingual transliterators. A possible explana- that the bilingual transliteration method designed tion to this gap will be given in the next section. to be adaptive to Russian loanwords successfully Compared to Aylandirow, an extensive rule- predicted the language and transcribed them. based transliterator, CER (i.e., ACC also) in tt- Table3 illustrates the number of error words cat- ru hybrid was slightly better by 0.002 points. In egorized with respect to the transliteration rules (tt- LCS F-Score, in contrast, Aylandirow has the same based and tt-ru hybrid). We can see from the table score as our tt-ru hybrid. This fact means that Ay- that, among the error words observed in tt-based landirow returned transliterations somewhat closer ( (T )), 336 words were correctly transliterated by to the gold data than our tt-ru hybrid. tt-ru hybrid. Note that the perfection is not necessarily the Examples of successful transliteration in our tt- ultimate goal of this transliteration. As described ru hybrid are zakon and sovet shown in the upper in detail in Section 7, there may be several ways of example of Table2. The tt-based transliteration spelling in actual language use, particularly of the converted them as zaqon and sowet, while tt-ru spelling of proper nouns. This variance in spelling hybrid correctly returned zakon and sovet. This foreign words is common among languages. shows that tt-ru hybrid successfully identified the language, since they are Russian loanwords. 7 Analysis However, tt-ru hybrid wrongly transliterated 116 The results illustrated that the bilingual translit- words that were correct in tt-based; for example, erators generally have higher accuracy than the in the lower example of Table2, the underlined transliteration soklanıp in tt-ru hybrid is a translit- 9Although we mentioned FinTat in Section 2, we excluded it from the evaluation because it is based on a spelling system eration error, where the correct word form is so- slightly different from 2013Latin. qlanıp. Because the language classifier identified 136 Source RF Zakon qygaruqylar sovety prezidiumy utyryxynda katnaxty. tt-based RF Zaqon çıgaruçılar˘ sowetı prezidiumı utırı¸sındaqatna¸stı. tt-ru hybrid RF Zakon çıgaruçılar˘ sovetı prezidiumı utırı¸sındaqatna¸stı. Source Һәr lqy tuktap, anyң hozurlygyna soklanyp kitә. tt-based Här yulçı tuqtap, anıñ xozurlıgına˘ soqlanıp kitä. tt-ru hybrid Här yulçı tuqtap, anıñ xozurlıgına˘ soklanıp kitä.

Table 2: Examples of successful and unsuccessful transliterations in tt-ru hybrid based on subword tokenization. The underlined words are error words (Russian loanwords). The upper sentence is an example where tt-ru hybrid can correctly transliterate the underlined words while tt-based cannot. The lower example is, on the other hand, tt-ru hybrid mistakenly transliterates the underlined word that is correctly transliterated by tt-based.

set (total 5,261 words) # words ru words CS words V (T ) 552 V () 332 # accuracy # accuracy V (T ) ∩ V (H) 216 speak.tatar 805 0.798 455 0.752 V (T ) \ V (H) 336 TTT 258 0.256 175 0.283 V (H) \ V (T ) 116 Aylandirow 738 0.731 461 0.762 Table 3: Comparison of the number of error words tt-based 471 0.467 294 0.486 (without duplication) between the monolingual tt- tt-ru hybrid 788 0.781 464 0.767 based transliteration and our proposed tt-ru hybrid transliteration. V (T ) is the set of error words observed Table 4: A comparison of performance in Russian CS in tt-based, V (H) in tt-ru hybrid, V (T )∩V (H) in both words. The left column (ru words) demonstrates the tt-based and tt-ru hybrid, V (T ) \ V (H) is the set of - number of correctly transcribed words that contain Rus- ror words observed only in tt-based, and V (H) \ V (T ) sian and its accuracy that is given by dividing by the only in tt-ru hybrid. total Russian words (1,009 with duplication). The right column (CS words) contains the number of correct tran- scriptions out of 605 words (with duplication) and its the first subword as Russian, the Russian translit- accuracy with respect to intra-word CS words. eration rule was applied, whereas in fact it is not a Russian loanword. language detection is expected to show a stable per- As for the high LCS F-score in Aylandirow formance to any Russian words; in contrast, rule- (0.994), it implies that Aylandirow is good at cor- based systems such as Aylandirow are less flexible rectly transcribing frequent words including Rus- to unknown Russian words, which, in effect, exist sian loanwords. Because Aylandirow is strictly infinitely in natural languages as hapax legomena. rule-based without automatic language detection, it can easily suffer from the rule coverage problem; 8 Conclusion for example, the word taksi (taksi, a Russian loan- In this paper, we proposed a new transliteration word) was mistakenly transliterated as taqsi. system that converts Cyrillic Tatar to Latin Tatar. As the comparison of performance with respect Taking into account the facts that different translit- to Russian CS words in Table4 shows, tt-ru hy- eration rules are applied to Russian loanwords and brid demonstrated higher accuracy in transliterat- that intra-word CS is frequently observed, our pro- 10 ing words with Russian morphemes . In particular, posed method involved language identification for the tt-ru hybrid’ performance adaptive to Russian each subword. Even though Tatar resources avail- morphemes is clearly visible in the accuracies of able at hand for training were limited, the results transcribing words containing Russian, where tt-ru were significantly better than existing translitera- hybrid scored 78.1% and tt-based 46.7%. tion tools. The simple architecture of language Considering that Russian CS in Tatar may arbi- detection employed in this approach is language- trarily occur, our proposed method with automatic agnostic does not need detailed analyses such as syntactic parsing and POS tagging, our method 10In this respect, speak.tatar scores the best for Russian words. This is merely because its transliteration rules are is applicable to other low-resource languages that designed to work well for Russian words, and, in contrast, its have intra-word CS. performance to Tatar words is poor as shown in Table1. 137 References for code-switching research. In 2020 6th Inter- national Conference on Advanced Computing and Elena Alvarez-Mellado. 2020. An annotated corpus Communication Systems (ICACCS), pages 136–141. of emerging anglicisms in Spanish newspaper head- lines. In Proceedings of the The 4th Workshop Armand Joulin, Edouard Grave, Piotr Bojanowski, and on Computational Approaches to Code Switching, Tomas Mikolov. 2017. Bag of tricks for efficient pages 1–8, Marseille, France. European Language text classification. In Proceedings of the 15th Con- Resources Association. ference of the European Chapter of the Association for Computational Linguistics: Volume 2, Short Pa- Tamali Banerjee and Pushpak Bhattacharyya. 2018. pers, pages 427–431, Valencia, Spain. Association Meaningless yet meaningful: Morphology grounded for Computational Linguistics. subword-level NMT. In Proceedings of the Sec- ond Workshop on Subword/Character LEvel Models, Dinar Korbanov. n.d. Aylandirow. Available pages 55–60, New Orleans. Association for Compu- at: http://aylandirow.tmf.org.ru [retrieved 29 March tational Linguistics. 2021].

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139 A Appendix

Cyrillic Latin (Tatar) Latin (Russian) Cyrillic Latin (Tatar) Latin (Russian) a a a u, uw, w u b b b f f f v w v h x g g, g˘ g NA ts d d d ç e e, , yı e x ¸s ¸s  NA yo w NA ¸sç  j j  —— z z y ı ı i i i ~ ——  y y e, ’ e k, q k  , yü, yuw, yüw yu l l , yä ya m m m ә ä NA n n n ө ö NA o o o ү ü, üw, w NA p p p җ c NA s s s ң ñ NA t t t һ h NA

Table 5: Tatar’s Cyrillic–Latin correspondence for Tatar- and Russian-origin words. NA (not applicable) means that the letter does not appear in the language. An -dash means that the letter is ignored in Latin transcription.

original RF Prezidenty Vladimir Putin Rossi mөselmannaryn izge Ramazan gold (Latin) RF Prezidentı Vladimir Putin Rossiyä möselmannarın izge Ramazan speak.tatar RF Prezidentı Vladimir Putin Rossiyä möselmannarın izge Ramazan TTT RF Prezidentı Wlädimir Pütin Rössiyä möselmannarın izge Ramazan Aylandirow RF Prezidentı Vladimir Putin Rossiä möselmännarın izge Ramazan tt-based RF Prezidentı Wladimir Putin Rossiyä möselmannarın izge Ramazan tt-ru hybrid RF Prezidentı Vladimir Putin Rossiyä möselmannarın izge Ramazan

Table 6: An example of comparison of transliterations. The sequence on the top is the original corpus sentence in Cyrillic, below which is the Latin counterpart manually transcribed. The three sentences in the middle row are transliterations by the previous tools. The first sentence in the bottom row is the monolingual tt-based translitera- tion, and the second is transcribed by our proposed method.

140