Quote from Master6651 >>

My question about the spell: Does the caster "identify" the language? Like imagine a sign written in orcish. If he casts the spell and on that sign he can read "Only our kind shell enter", does he know that "our kind" must be orcs, or is that a seperate check?

I would say no, you don't automatically know what language something is written in just because you can magically understand it.

thanks

Is there any way to prevent the use of this spell on a piece of written text?

I am trying to come up with a puzzle for an upcoming campaign and it relies on an uncommon language containing the key for decrypting the puzzle. It feels super lame to try and limit the players and their solutions to these puzzles, but the whole point is to get them to socialize with the NPCs and find someone that does understand the language, and this spell kinda undoes that whole idea. 

Quote from RamenExpress >>

Is there any way to prevent the use of this spell on a piece of written text?

I am trying to come up with a puzzle for an upcoming campaign and it relies on an uncommon language containing the key for decrypting the puzzle. It feels super lame to try and limit the players and their solutions to these puzzles, but the whole point is to get them to socialize with the NPCs and find someone that does understand the language, and this spell kinda undoes that whole idea. 

You could always have it so the writing is enchanted in some way to resist Comprehend Languages. It is, as they say, your world.
I would however suggest something else, such as putting the writing somewhere that the characters cannot touch the writing, as that is required per the spell's text.
That or I might try to find another reason to need to speak to an NPC, like the answer to a riddle that can't be gleaned from the literal meaning alone, relying on some metaphor (in the language in question) that the player characters might not be aware of.
This spell has some inherent limitations you could make use of if you want to.