As written the Linguistics skill doesn't let you make checks to understand a spoken language you don't know. This is silly; and it is going to come up often in Falling Star, because the players are off in a benighted corner of nowhere and have limited opportunities to learn languages.
As a house rule: after chargen, you can only learn a language at level-up if you have a teacher, or had one during the previous level at some point. The teacher needs to know the language in question; needs to share at least one language in common with you; and needs to be available for a total of a week. This does not require any special activity on the part of you or the teacher, you just need to be frequently close enough to communicate.
As another house rule: if you hear speech in a language related (see below) to one you know, you can make a linguistics check to understand it. A DC 15 check gets you 1d3 words; a DC 25 check gets you a paraphrase. If you want to communicate with someone in a language related (see below) to one you know, a DC 15 check lets you say one word and a DC 25 check lets you say a two-word phrase. You cannot retry a failed check with the same message until 24 hours have elapsed. These two types of checks are free actions. You can assist someone else with one of these checks only if you would be eligible to make the check yourself. If you know more than one related language, you get +2 on the check for each additional related language you know beyond the first.
Here are the relations between many of the relevant languages of Golarion. Touching bubbles or bubbles connected by lines are related.