  We tackle the task of automatically discriminating between human and machine
translations. As opposed to most previous work, we perform experiments in a
multilingual setting, considering multiple languages and multilingual
pretrained language models. We show that a classifier trained on parallel data
with a single source language (in our case German-English) can still perform
well on English translations that come from different source languages, even
when the machine translations were produced by other systems than the one it
was trained on. Additionally, we demonstrate that incorporating the source text
in the input of a multilingual classifier improves (i) its accuracy and (ii)
its robustness on cross-system evaluation, compared to a monolingual
classifier. Furthermore, we find that using training data from multiple source
languages (German, Russian, and Chinese) tends to improve the accuracy of both
monolingual and multilingual classifiers. Finally, we show that bilingual
classifiers and classifiers trained on multiple source languages benefit from
being trained on longer text sequences, rather than on sentences.
