Each site is permitted three custom off-topic close reasons. I'd like to propose one for questions about specific languages.
My proposed text (leave comments to suggest changes):
Questions about grammatical rules or specific characteristics of a language are off-topic here. A language-specific site* is the appropriate place for these questions, or if such a site does not yet exist, you can propose one on Area 51.
*This text could/should link to a meta post detailing all of the language-specific sites which currently exist on SE. (Or a link to such a listing on stackexchange.com, but I can't find such a list, other than this, which seems too broad).