Due to this, I believe Linguistics skill, despite having specialties, should be treated as the Trde skill in the way knowing one specialty at any level should not give you skill 0 in all other specialties (in this case, other languages). At most, it could give it in related languages, if the referee wants to enter in this level of detail.
e.g. as I can talk several languages (as English, not being my native one), and so I must have some Linguistics skill in Traveller terms, that does not mean I can understand a single word in most world languages...