Working in a bilingual workplace that deal with some suppliers speaking English and some speaking French the fact that I can’t change the autocorrect language during writing is very inconvenient in a otherwise almost perfect mail client
The best solution for me would be a language selector at the right of the writing style tool, so you can choose easily, maybe this selector could be hidden if there is only one language selected in the settings for example.
Another less friendly option but still good enough (but maybe lower development effort) would be a selector in the context menu when right clicking the text
A very good example of this is the mailbird client:
