According to Google, there are some 57 languages/dialects that are officially supported by stock Android. Of course, since Android is open source, there is nothing to stop a device manufacturer, or cell service provider, or even end user from adding support for new languages/dialects.
Now, just because it is possible, that doesn't mean that it has been done, or at least that it has been done on a scale large enough to make an impact. For example, I would likely not spend my time translating my app for a language that 5 people have manually added to their own devices, however if a manufacturer of devices has decided to add support for an additional language that is now in the hands of some thousands of users, that may be a useful translation to make even though it is not officially supported by the AOSP.
So, is there any way to find this out? Or if not officially, is there any anecdotal evidence out there that this type of thing is going on?