You can point directly to a localized by doing /<icodecode> like /fr (case-sensitive).
The list of supported languages is available in the User Guide.
Helping with translations
The localization is handled through Crowdin. You will need to get a translator account to get started.
Once your translations are added, someone will have to approve if you are a proofreader, go ahead and approve them. Then send us a comment on the crowdin message board to refresh the build.