One way to have your "dictionary" include spellings from other languages is to translate the document into the language you desire. This will create a copy of the document in the language you choose.

Though this way may produce some weird results, I find that once translated the "spell check" does provide corrections with the translated language, though it will also still correct English spellings.

An example of one weird result I've seen while translating from an English document to a German one had Sprechen sie deutsch translated to Do you speak English. Though, it appears this may have happened because the document had been translated multiple times between languages before translating it back to English, where I added the line which spell check corrected it for German spelling.

Another weird result was the translation failing to separate words causing the "spell check" to flag the word, but it did give the correction in the language it was translated to.

Hope this helps.

Short answer

Google documents doesn't include a feature like Gmail filters that automatically process incoming messages, but it could be extended through add-ons and Google Apps Script.

Alternatives

Google Apps for Education includes Google Classroom which help teachers to manage assignments and include features to help teachers to manage files submitted by students.

References

Extend Google Docs, Sheets, and Forms with Apps Script - Docs editors Help