Avoid accidentally uploading different language to a translation

**Is your feature request related to a problem? Please describe.**
With upload the language of the file is not checked. In case user is manually uploading more files, it is easy to mess up and upload different file.

**Describe the solution you'd like**
Weblate should try to validate whether language of the uploaded file matches. In some file formats this information is present (XLIFF) so we should utilize it.

I'm not sure if we should try to detect language from the filename as well, that looks too fragile, but it might be the only way for formats which do not expose this.

There also needs to way to override the detection in case it is wrong.