  Techniques in causal analysis of language models illuminate how linguistic
information is organized in LLMs. We use one such technique, AlterRep, a method
of counterfactual probing, to explore the internal structure of multilingual
models (mBERT and XLM-R). We train a linear classifier on a binary language
identity task, to classify tokens between Language X and Language Y. Applying a
counterfactual probing procedure, we use the classifier weights to project the
embeddings into the null space and push the resulting embeddings either in the
direction of Language X or Language Y. Then we evaluate on a masked language
modeling task. We find that, given a template in Language X, pushing towards
Language Y systematically increases the probability of Language Y words, above
and beyond a third-party control language. But it does not specifically push
the model towards translation-equivalent words in Language Y. Pushing towards
Language X (the same direction as the template) has a minimal effect, but
somewhat degrades these models. Overall, we take these results as further
evidence of the rich structure of massive multilingual language models, which
include both a language-specific and language-general component. And we show
that counterfactual probing can be fruitfully applied to multilingual models.
