Suggest declaring available languages

[tstephen]

There are almost 200 two letter language codes in ISO 639. There are several scenarios in which knowing which ones are intended to be available may be useful, for example:

1. Rendering process diagrams in all available languages by just checking if there is at least one translation element for each language is not very efficient ;
2. Validating which languages are expected and therefore detecting missing translations is much more helpful to users than just having to check every element.

In fact, I notice Trisotech already include a triso:translationLanguagesattribute on the definitions element. I suggest we bring this into the existing extension.