It also happens with documents going back and forth between different computers, and I blame MS Office. I edit manuscripts in LibreOffice on my end, and tag every foreign-language word as its respective language for the sake of spellcheck and proper hyphenation. Then I send the .docx to the other party. When they send a fresh version back to me, I see that all those foreign-language words were simply restored to some default language.

Have you read the manual? I haven't used MS Word for many years (luckily) but the last time I did I actually did figure out how languages work once and for all. It made sense and it's quite powerful. I also figured out how styles work and that you should never press the bold/italic etc buttons, only use styles.