Reply To: Update 2.5.4 – Lost Languages Files
You’ll need to download the translations manually per the details here:
https://codex.bbpress.org/bbpress-in-your-language/
https://translate.wordpress.org/projects/bbpress/2.5.x
Save them into /wp-content/languages/plugins
With some luck these two direct links will get you the .mo and .po. Make sure you rename both files so they are named bbpress-fr_FR.mo and bbpress-fr_FR.po respectively.
The reason I asked for the language you are using is because if the language translation is 100% translated then bbPress translations are delived to you automatically by the same WordPress Core ‘Automatic Updates’ mechanism, sadly bbPress French translations are not at 100%. (Technically they are at 100% translated but there are 228 strings waiting to be approved.)
