As I'm writing my campaign, I have run into a problem. How do I make my players interact with an unknown or foreign language? Is there a way I can make them learn the language before gaining the language skills? I'm looking for advice on this and anything helps, even if it's a vague idea.

EDIT: Upon running into confusion, here's a better explanation of what I mean. I want my players to learn learn languages, not level up and gain the sudden ability to speak and comprehend a language. How can I incorporate that into my campaign?

I’m unclear on the question. You just put in whatever language you want, and then have the characters run into people speaking those languages. 
If you mean, how do you make sure the characters speak the appropriate languages, you warn them during character creation “You all probably want to be sure someone speaks goblin.”

Or you don’t worry about it. If the characters don’t have the appropriate proficiency, that’s a problem for them to solve. It can be a side quest to find a person to translate for them. 

Also, comprehend languages is a low level spell. There’s warlock with eyes of the rune keeper for reading things. Tongues is pretty low level also. There’s lots of strategies. 

Also as a downtime activity they can work on learning a language... obviously this takes a bit of time but you can have them progressing towards it. Moatly though they'll have some form of magic that helps them understand it and/or speak it.

characters can run into npc speaking those languages or get item on those languages or just investigate place with writings on those languages, they can manage get info on unknown language even without language learning as story point - via spells. There is also 3rd party publishers items which allow speak unknown languages (eg vault of magic page 150 by kobold press had rare wondrous item called language pyramid specifically for this)

And it's all up to you as DM to decide

Xanathar's has a Downtime Revisited section, which includes simple rules for learning a skill or language. It has a timeline and a cost. I use it all the time and let people break up the time as needed so the game doesn't stop.

I love making language useful, but be ready for character abilities that circumvent it (part of the fun).

Where is the Downtime Revisited found? That seems like something I should look at.

Quote from CofeWaror4 >>

Where is the Downtime Revisited found? That seems like something I should look at.

 Here: https://www.dndbeyond.com/sources/xgte

Read the "Downtime Revisited" section. 

You can always have a non-combat NPC follower show up and teach them how to speak the NPC's "native tongue". 

Or, if you like creepier options, have a mindflayer set up a "New Languages, 5 copper" stand. Whenever they pay it gives their brains a tentacle licking and implants new language skills. 

How involved do you want it to be?   A simple way to do it is to create a new magic item that grants people the ability to learn a specific language.   Have some artificer/wizard living in the area be selling them to various communities of creatures.  

Or you could make a new rule about spending x amount of time to learn a language from a willing teacher.

I made a chart that shows how the different languages are related and by passing a wisdom check if you know a related language you can pick out common words.

eg. character who speaks Sylvan can make a wisdom check to try and understand goblin because goblins originally come from the feywild.

that might be useful