Are there commercially available devices that support more languages than those officially supported by Google?
From prior experience in building AOSP before, the language strings for all aspects of the system, are stored in xml file, found in the AOSP source framework/base/core/res/res. Inside that directory in the source, there's values-XXX/strings.xml and values-XXX/do-not-translate.xml, where XXX is the ISO country code based on the standard guidelines. This gets built and generated into framework-res.apk found in the ROM's live system under /system/framework.

On an existing ROM, its possible to 'sort of' add translations into it by decompiling it but the effort would not be worth it as the XML data is compiled and "gets lost in translation". I say 'sort of' very loosely, as either the outcome would be it may work or it may not work, but the effort would be big and possibly break existing core Android functionality.

You can most certainly add a language add-on for the keyboards but that's about it.

From the aspect of developers, where localization is concerned, the principle is pretty much the same as in the AOSP source. So if the developer wants to add in French language support, he/she can simply create a values-fr/strings.xml and shove in there, the french translations and so on. Of course, there will be variants such as Canadian French, Swiss French, for example. So their files, would be values-fr-rCA/strings.xml, values-fr-rCH to cite the example.

This is where CM has the most languages support and are in a state-of-flux due to users spotting mis-translations etc. 

Only Google provides the core languages - such as Italian, French, German, Spanish, English (both US/UK) to name but a few.