allow override of language for checking a file / selection

[GitMensch]

Is your feature request related to a problem? Please describe.
I commonly work with files that have multiple languages in them. Using magic comments #21 helps for files we actually edit, possibly for longer time, but it won't help for read-only files or for "just a quick look".

Single strings may be in different languages, especially with quoting. There's no option to select a language for ltex.checkSelection at all, I guess it uses the configured "main" language.

Describe the solution you'd like
Add an optional parameter to ´ltex.checkCurrentDocumentandltex.checkSelection (or new commands ´ltex.checkCurrentDocumentWith and ltex.checkSelectionWith) to allow override of the language used to check.

Describe alternatives you've considered
Have multiple documents open with a magic comment, then paste the parts to check there, then do the check. Works, but takes quite some time.

Additional context
None.