This would be great if you wanted to talk to yourself without anyone understanding what you’re saying, for example, or if you didn’t want anybody to know what you were writing down. It might also be useful if you enjoy lots of foreign language media (see music, tv shows, movies) and want to experience the “original” rather than through dubs and subs.
Personally, I would go for either Russian, German, or Estonian, all three of which are because I frequently listen to lots of music that are in those languages
If we’re talking city wide, then I doubt there are many fluent French speakers. So I’d Probably choose French just to not use it.
The trick is to learn french, then pretend not to know it if any french try to use it with you, out of spite.