The bug appeared when we updated from 4.0.3 to 4.1.0.
As we are in a hurry, we reverted back to 4.0.3.
It seems that, as ezwebin was installed first (english by default) and we created classes and contents in french, the database was a mix of both languages.
I have my idea on it, but can you explain us what means exactly language_mask or lang_mask, compared to language_id?
