In this test project, one language is supported by the translation provider and one is not.

The translation provider is disabled for both languages in the project after trying to create the project step 9 - "Preparation".

This is the warning message displayed for the multilingual project with the two languages.

Is there a way to warn the user before this point that the language is not supported?

E.g. the "Translation Resources" step in "Create a New Project". This is where a TM or MT plugin is disabled in a project when the language pair is not supported. Only when it's a single language pair (not multilingual project)

Can we force the same warning for multilingual projects as we get in a monolingual project?

Also, would it be possible to allow the plugin to work for some languages in a multilingual project and not others?