Even if you set your fusionpbx to some other language, you might be wondering why the conference still uses English for prompts.
The reason for this is that the conference uses settings from the conference profile which seems to default to English but it's quite easy to fix.
Keep in mind, this setting applies to all domains!
Go to Apps -> Conference Profiles and edit the profile you want.
If you want different settings (different Domains for example), you could copy the profile and create separate profiles with different languages.
Click + to add a new value
and add a parameter sound_prefix with the path to your language files
save and you're done!