I believe this is common for every non-English user: texts typed will contain both native words and English words. Right now Firefox only supports single language per text field and doesn't even auto-switch between them based on user input without add-ons. This is different to the experience offered by literary every other software product: MS Word will proof each word independently based on active keyboard, default Windows 10 proofing works in similar fashion in every native Windows app, Chrome will do the same too. Firefox instead highlights everything which is extremely annoying and unpleasant. I do understand that such feature might have a very big impact on source code, but as it stands today, spell checker in Firefox is unusable for non-English users.