The debug locale directories needs to exist either as db_LB or as the defined debug_locale even though it appears to be generated on the fly when using i18n as a middleware.
So looking at 68c02ad it looks like the debug locale used to be generated. Unless I'm missing something it looks like gobbledygook is used on the fly means we're missing the debug locale for client-side only apps because there's no .po to generate JS from.
So one workaround here would be to use gobbledygook client-side in the same way as the middleware.
For the client-side part of this I'm currently in the process of looking at bowerising gobbledygook.
It'll then probably make sense to make a bower package for the client-side gettext and format utils (which can then use gobbleygook as a client-side dep) which I'll create new moz repo for.