title_active_language() which basically relies on the content / interface language set if not actively set otherwise.
This means it works fine for e.g.
en/node/1/edit/en because the interface language is the same as the translation language.
However, it fails if
de/node/1/edit/en is used even though this is a perfectly fine / allowed / reasonable path to edit a translation (without having to switch the interface language too).
The issue leads to overwritten titles in the interface language.
title_active_language() use the form language of the entity translation handler.
Another approach would have been to use the form language of the entity translation handler only if one is explicitly set, but unfortunately the entity translation handlers (currently) provide no way to check that.
See if tests pass - locally I get very random test failures and I can't figure out why (independent if the code base)
I've added a new test which uses another interface language instead the translation language as path prefix. But I don't know yet if this test is valid the way it is.
User interface changes