Input languages
In the previous versions of Firefox, when selecting a language of input, all the other languages were unselected. This was a much better option than today's, where they remain selected, allowing to check in several languages simultaneously. The problem with that is that when a word exists in both languages with a small difference (accent, two letters reversed or something like that), if there is a spelling mistake, it may not appear. In the example below, "defense" should be with an accent: "défense".
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You can currently select multiple dictionaries for spell checking, so in your case probably best is to ensure you only have one selected at the time to avoid confusion.
I speak enough languages to know that that'll be the case for all the languages written with the same, or almost the same, alphabet so it's a BAD "current" situation, not a good one at all. It may have seemed like a good idea (avoid having to change language) but it in fact doubles the trouble.
I don't think that there is an easy solution of how to deal with words that appear in more than one dictionary as it is possible that you would want to quote a sentence in a language other than the main language you currently use and you would have to flag words that appear in multiple dictionaries what can be quite annoying at times.