This functionality is handled by the operating system in Scriv on the Mac, and so is (as things stand) outside the control of Scrivener. I’m not sure how things work on the Win platform.
It does seem like a desirable thing and I wish for you some happy solution. It is interesting to think about how the feature you are imagining would perform. It seems like it would be prone to a certain kind of error: it would give a clear pass to anything that was a word in either language no matter which language you were actually writing in. So, while you might have mistyped a phrase in English ‘cor values’ instead of ‘core values’, your bilingual spell-checker would still give it a pass if ‘cor’ was a word in the other language. To be really accurate/reliable, a bilingual checker would have to have some way of knowing which spans of text were in which language. Still, I imagine having the function would probably still be worth it for you in spite of such errors!
Chrome and Firefox works that way, having two languages enabled at the same time. As far as you’re aware of it, I believe it’s not a problem. It’s far more annoying to have to switch on and off the real time spell checking or to switch languages.
It would be manageable if you could change these properties from a easy accessible tool button and pull down menu, but as now you are forced to navigate to a settings page which takes some effort and mouse clicks each time.
In the same editing space, with spell check for both? I don’t see how that’s possible, even theoretically.
Well, you would have to insert language identifiers like <language=“EN-en”>word</language> for every word in your manuscript.
<language=“EN-en, FR-fr”>word</language> when a Word is in the dictionary of both languages and spelled alike.
Okay, I stand schooled then!
Apple gives directions here for getting multi-language spell-check on MacOS:
So, this is clearly already doable on the Mac side.
Nisus Writer Pro (Mac only) let’s you mark stretches of text as being in a particular language and applies the appropriate spell check dictionary if you set it up in Preferences. It’s also based on the Apple TextKit, so it’s possible, though I don’t know how difficult it is. One reason why I have check spelling turned off in Scrivener … I do it in NWP after compiling.