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Title: Should All Cross-Lingual Embeddings Speak English?

Paper content:
Most of recent work in cross-lingual word embeddings is severely Anglocentric.
The vast majority of lexicon induction evaluation dictionaries are between English and another language, and the English embedding space is selected by default as the hub when learning in a multilingual setting.
With this work, however, we challenge these practices.
First, we show that the choice of hub language can significantly impact downstream lexicon induction performance.
Second, we both expand the current evaluation dictionary collection to include all language pairs using triangulation, and also create new dictionaries for under-represented languages.
Evaluating established methods over all these language pairs sheds light into their suitability and presents new challenges for the field.
Finally, in our analysis we identify general guidelines for strong cross-lingual embeddings baselines, based on more than just Anglocentric experiments.
Continuous distributional vectors for representing words (embeddings) (Turian et al., 2010) have become ubiquitous in modern, neural NLP.
Cross-lingual representations (Mikolov et al., 2013) additionally represent words from various languages in a shared continuous space, which in turn can be used for Bilingual Lexicon Induction (BLI).
BLI is often the first step towards several downstream tasks such as Part-Of-Speech (POS) tagging (Zhang et al., 2016) , parsing (Ammar et al., 2016) , document classification (Klementiev et al., 2012) , and machine translation (Irvine and CallisonBurch, 2013; Artetxe et al., 2018b; Lample et al., 2018) .
Often, such shared representations are learned with a two-step process, whether under bilingual or multilingual settings (hereinafter BWE and MWE, respectively) .
First, monolingual word embeddings are learned over large swaths of text; such pre-trained word embeddings, in fact, are available for several languages and are widely used, like the fastText Wikipedia vectors (Grave et al., 2018) .
Second, a mapping between the languages is learned, in one of three ways: in a supervised manner if dictionaries or parallel data are available to be used for supervision (Zou et al., 2013) , under minimal supervision e.g. using only identical strings (Smith et al., 2017) , or even in a completely unsupervised fashion (Zhang et al., 2017; Conneau et al., 2018) .
Both in bilingual and multilingual settings, it is common that one of the language embedding spaces is the target to which all other languages get aligned to (hereinafter "the hub").
We outline the details in Section 2.
Despite all the recent progress in learning cross-lingual embeddings, we identify a major shortcoming to previous work: it is by and large English-centric.
Notably, most MWE approaches essentially select English as the hub during training by default, aligning all other language spaces to the English one.
We argue and empirically show, however, that English is a poor hub language choice.
In BWE settings, on the other hand, it is fairly uncommon to denote which of the two languages is the hub (often this is implied to be the target language).
However, we experimentally find that this choice can greatly impact downstream performance, especially when aligning distant languages.
This Anglocentricity is even more evident at the evaluation stage.
The lexica most commonly used for evaluation are the MUSE lexica (Conneau et al., 2018) which cover 45 languages, but with translations only from and into English.
Even still, alternative evaluation dictionaries are also very English-and European-centric: Dinu and Baroni (2014) report results on English-Italian, Artetxe et al. (2017) on English-German and English-Finnish, Zhang et al. (2017) on Spanish-English and Italian-English, and Artetxe et al. (2018a) between English and Italian, German, Finnish, Spanish, and Turkish.
We argue that cross-lingual word embedding mapping methods should look beyond English for their evaluation benchmarks because, compared to all others, English is a language with disproportionately large available data and relatively poor inflectional morphology e.g., it lacks case, gender, and complex verbal inflection systems (Aronoff and Fudeman, 2011) .
These two factors allow for an overly easy evaluation setting which does not necessarily generalize to other language pairs.
In light of this, equal focus should instead be devoted to evaluation over more diverse language pairs that also include morphologically rich and low-resource languages.
With this work, we attempt to address these shortcomings, providing the following contributions:
• We show that the choice of the hub when evaluating on diverse language pairs can lead to significantly different performance (e.g., by more than 10 percentage points for BWE over distant languages).
We also show that often English is a suboptimal hub for MWE.
• We identify some general guidelines for choosing a hub language which could lead to stronger baselines; less isometry between the hub and source and target embedding spaces mildly correlates with performance, as does typological distance (a measure of language similarity based on language family membership trees).
For distant languages, multilingual systems should in most cases be preferred over bilingual ones.
• We provide resources for training and evaluation on non-Anglocentric language pairs.
We outline a simple triangulation method with which we extend the MUSE dictionaries to an additional 2352 lexicons covering 49 languages, and we present results on a subset of them.
We also create new evaluation lexica for under-resourced languages using Azerbaijani, Belarusian, and Galician as our test cases.
We additionally provide recipes for creating such dictionaries for any language pair with available parallel data.
With this work we challenge the standard practices in learning cross-lingual word embeddings.
We empirically showed that the choice of the hub language is an important parameter that affects lexicon induction performance in both bilingual (between distant languages) and multilingual settings.
More importantly, we hope that by providing new dictionaries and baseline results on several language pairs, we will stir the community towards evaluating all methods in challenging scenarios that include under-represented language pairs.
Towards this end, our analysis provides insights and general directions for stronger baselines for non-Anglocentric cross-lingual word embeddings.
A Does evaluation directionality matter?
We also explored whether there are significant differences between the evaluated quality of aligned spaces, when computed on both directions (src-trg and trg-src).
We find that the evaluation direction indeed matters a lot, when the languages of the evaluation pair are very distant, in terms of morphological complexity and data availability (which affects the quality of the original embeddings).
A prominent example, from our European-languages experiment, are evaluation pairs involving Az or Be.
When evaluating on the Az-XX and Be-XX dictionaries, the word translation P@1 is more than 20 percentage points higher than when evaluating on the opposite direction (XX-Az or XX-Be).
For example, Es-Az has a mere P@1 of 9.9, while Az-Es achieves a P@1 of 44.9.
This observation holds even between very related languages (cf. Ru-Be: 12.8, Be-Ru: 41.1 and Tr-Az: 8.4, Az-Tr: 32.0), which supports our hypothesis that this difference is also due to the quality of the pre-trained embeddings.
It is important to note that such directionality differences are not observed when evaluating distant pairs with presumably high-quality pre-trained embeddings e.g. Tr-Sk or Tr-Es; the P@1 for both directions is very close.

assistant: text: The choice of the hub (target) language affects the quality of cross-lingual embeddings, which shouldn't be evaluated only  on English-centric dictionaries.