Add check: CachePrefixIsSet

[michapietsch]

When an in-memory caching is used in a shared hosting environment (e.g. because there's an additional staging system on the same server) and no cache prefix is set and Laravel sites have the same app name configured, Laravel will fall back to a default cache key prefix which leads to shared cache key access across sites.

I'm sorry, I have no idea why the language files are completely rewritten in the commit. Of course I just added the relevant lines and that's also what git diff indicated.