I have this same problem with Microsoft Office on Mac and Windows.
By far my least favorite issue is the constant battle I have with locale – I’m not sure if the document creators local was set to EN-US or if my system config is not correct, but every time I try and force the document to EN-AU it seems to switch back seemingly on a per-section basis or something.
If I type in French, even if I type no English and paste nothing into the document, certain sections seemingly randomly switch to English thus putting damn red squiggles under nearly all my French words.
I don’t understand how this can work so poorly. If I say the document is in French, I mean it and I mean it for the whole document. I continue to mean it even if I do type one apparently (to MS Office) English word.