[kentlii]

Updating to 2.5 erased the translation of the forum. Is that normal?
So you should save the translation before updating and then upload it again?

[Stephen Edgar]

You should have your translations in /wp-content/languages/bbpress/ not in the /wp-content/plugins/bbpress/languages/ folder.

See this for further information, also if your translations are at 100% for your language there is a very high chance you will now get this automatically updated when new translations are added/corrected etc.

https://codex.bbpress.org/bbpress-in-your-language/