Bug 14100: Better errors handling

Bug 14100: Better errors handling

This patch mainly improves the errors handling.
It's now not possible to add several translations for the same entity-code-lang
combination.

It also:
- wording: Translate for other languages => into
- fixes encoding issues on add/update (was just display)
- add entity and code to the title of the pop-up window

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>