Although you can import books written in languages other than English, yes, there are limitations.
Currently Atticus doesn't fully adapt to right aligned languages (such as Arabic), though you can right align text, so if it's imported right-to-left, this will work fairly well. Features such as the drop caps simply won't be properly functional.
When it comes to exports and how well that works, that will be somewhat dependent on the language. ePubs are very adept at converting to most languages however PDF files have more trouble with certain characters. Languages that use Latin-based characters should be fine, however, non-Latin scripts may not export correctly if you're only using the regular body fonts. This is because the characters need to be specifically programmed into the font family used, which isn't the case for most fonts sadly.
There is an option which may work, but it depends on how you need to add the foreign languages.
If you apply a subheading style to a complete line of text or full paragraph, you can set that heading style to be a Google font in the language of your choice. This video walks you through the steps for that process: https://www.youtube.com/watch?v=PXwEk_7SuyM
Hopefully that helps!