Bug 917480 just landed in mozilla-central, and should show up in your next Nightly. This sizable chunk of work provides settings UI for selecting a locale within Firefox for Android.
If all goes well in the intervening weeks, Firefox 32 will allow you to choose from our 49 supported languages without restarting your browser, and regardless of the locales supported by your Android device. (For more on this, see my earlier blog post.)
We’ve tested this on multiple Android versions, devices, and form factors (and every one is different!), and we’re quite confident that things will work for almost everyone. But if something doesn’t work for you, please file a bug and let me know.
If you want more details, have a read through some of my earlier posts on the topic.