I'd like to add that you can use custom translation loaders, just like FoF Translate does. If you want to store your translations as CSV for example, that's totally doable. This also allows to generate translations from logic instead of a hard-coded list.
No matter your loader, performance shouldn't be much different. The translator gets cached and the backend translator as well as the frontend JS is not generated until new extensions are enabled/disabled or when manually clearing the cache. If those take long to generate on large communities, there's an extension available to generate assets server side through the command line instead of having the first visitor slowed down by the compilation.
Extension can add content to the locale-specific JavaScript, but this is not a way to extend the translator, this is only designed to localize other features, like for example date formatting.
