[BUG] GenerateFilesAfterTcaSave::getLanguages() won't respect SiteConfiguration (TYPO3 v9)

Since TYPO3 v9 introduced SiteConfiguration it's possible to enable various languages for different trees.

Now when a language is missing from a tree, that's available in an other tree GenerateFilesAfterTcaSave will try to generate static files for non-available languages and throw an Exception.

My proposed solution:
Use SiteFinder to fetch available Languages instead of all languages.

Unfortunately I'm unable to fork this Project to create a Merge Request, so here's a patch file: bugfix-115.diff