Thank you for your answer and the very quick fix !  I tested your new file and it works perfect now.

For my knowledge, Just a question... What is the preferred way for the Piwigo team to report bugs ?

1) Open a forum post and check with other users that this is a true Piwigo bug and not something related to our customizations and, only once the bug is verifed, create a bug report in Mantis.

2) Directly create a bug report in Mantis (even if it could appear after deeper review that this is not a Piwigo bug).

Piwigo 2.3.2
Extended descriptions 2.3.c
Personnal theme based on default (file thumbnails.tpl is not modified).

I am using the plugin Extended description to make my gallery multilingual. If I write something  like [lang=fr]description[/lang][lang=en]description in English[/lang] in the picture description, everything is displayed in the thumbnail title tag of the thumbnails page. I checked on the picture page. Here the language filter works correctly.

Does anyone reproduce this bug ?