[zimmermann.c]

With the

localization

plugin, text tranlation does not work: people still get the english text,
eg :
Hello!\n\nA new topic has been posted by [topic-author].\n\nTopic title: [topic-title]\nTopic url: [topic-url]\n\nTopic excerpt:\n[topic-excerpt]\n\nNotify message by bbPress Notifications

in addition, the Bbp extension (in that localization) says
Compatibility: Loading Issue: Author is using load_textdomain instead of load_plugin_textdomain function. This may break behavior of WordPress, because some filters and actions won’t be executed anymore. Please contact the Author about that.
Thanks for for helping me to find another way to get french texts
regards Charles

[treeflips]

This might be overkill but WPML might work.

[zimmermann.c]

We found the solution:
notification text is to be set in the forum settings, not via the localisation plugin