Linguistics is actually a specialty of the Language skill — see the table on page 65 and the end of the Language (Other)! skill description on page 70:
Specialties: [other Language], Linguistics
(Yes, this could have been clearer in the book!)
Before that it explains what Linguistics does, albeit very briefly. Basically, it does what linguistics means in the real world: it's the scientific study of how human language works, which is helpful in the game for documenting, analysing, and ultimately learning the mutated or completely unknown languages that will have developed during the 150 years the characters have been frozen.
There is no support for how to use Linguistics to accomplish this, only the note that it allows construction of a translation guide “given enough time”. The details are left up to the GM, so if you have a character with Linguistics trying to decode the language of a newly-discovered post-apocalypse community, you should work together to treat it as a medium-term project, just like constructing a reliable water supply or teaching the inhabitants basic sanitation and medicine would be time-consuming Morrow Team projects.
Aside, original (1st through 3rd edition) The Morrow Project doesn't include skills at all; however, there was a 3rd-party supplement put out by Chaosium by “H. N. Voss” that included a skill system, including a Linguistics skill. It did pretty much the same thing, but with a few more (somewhat unrealistic and brief) mechanical details for how long it would take to get how much percentage of skill in a new Language skill specialty.