Enable multiple languages at once

[RalfJung]

Is your feature request related to a problem? Please describe.
I switch between English and German a lot, so my "notes" workspace where I keep track of all my notes is a wild mix of documents in both languages. ltex lets me select a language per workspace, but it's a single language, so no matter what I select it will show tons of spelling mistakes for files in the other language. I don't want to add comments to each file indicating the language, that's just too much of a hassle. (And I may even switch the language mid-file accidentally.)

Right now the only option I see is to entirely disable ltex in that workspace.

Describe the solution you'd like
I'd like to set ltex.language to an array of languages. I am completely okay if that degrades the quality of the spellcheck, even reducing it to little more than a pure dictionary -- that would still be better than having no spellchecker at all!

This ability to select more than one language recently landed in Firefox and Thunderbird and it is extremely useful. Yes it might mean some typos slip through but it also means I spend less time configuring my spellchecker and that's totally worth it.

Describe alternatives you've considered
I guess there's always the alternative of doing nothing and declaring this "not supported". That would be unfortunate though. :)

Additional context
Nothing I can think of. Except for -- thanks for the extension, it's been very helpful already. :)