First, it's worth noting that the fully localized OS isn't quite ready yet, so even if you implement the strings files, you'll have to manage your own language switcher for now, local to the game.
I did do basic some testing of this in the past and it's easy enough to set up a translation system to switch Fonts and strings, for nonEN/JP strings. And, the EN/JP strings files do work properly (make sure you use double quotes in those strings files, I had some weird issues otherwise!)
Maybe you tried it already, but I don't see any way to hijack the SDKs custom strings file for additional languages, like "fr.strings" or something. The best you could do is store the key/value in a lua table somewhere for now, until further language support arrives.
I'd picture that as something like any time you're using drawLocalizedText(), to first check your game's global language setting and if it is not English or Japanese, use instead a drawText with a manual lookup/search to that French (or whatever current language) strings table. There may be a simpler way, but in general it will require managing it yourself for now.
Worth noting, there's an "options" menu type :addOptionsMenuItem() that allows you to have a multi-select option, so you could have as many languages as you like 