Is Language.dependsOn a UsedLanguage?

[enikao]

a) "Association class"

Pro:
References from one language to another are real references, just like references from a feature to its type
Con:
We don't have association classes in LIonWeb, so we cannot express M3 in itself.

b) UsedLanguage doesn't appear in M3, it's an serialization format issue

Pro:
Real references (like above)
Con:
Element present in every implementation not part of M3

c) Containment

Pro:
Can be represented in M3
We're using the more stable key instead of id
Con:
Not a real reference, i.e. breaks #144

Decision: Language.dependsOn is a regular reference to other Languages, no special treatment (variant b)