How often as a DM do you plant some clever clues in an ancient or forbidden language, eager to drop some lore or a puzzling mystery for your players, only to have Comprehend Languages or a similar spell (Eyes of the Rune Keeper any warlock?) rip away all the mystery?

The underlying premise is that these clues are to be deciphered eventually, otherwise why include them in the first place? Here are a couple of quick ideas for a player getting use out of their spell and the DM still keeping some secrets to be unlocked

Coded Messages
Honestly, this is a brute force approach, right up there with “The Gods Will It Not Be Read”.

Codes can be as simple as letters written on a belt that gets wrapped around a rod of a specific diameter, letter substitution, or even the Mad Magazine method of folding a page to create new letter combinations.

Historically speaking, coded messages where used by many, including spies, alchemists, and secret societies. The feat of Linguist is a bit of a work around depending on how much downtime is required for the decoding, slowing things down (and a metagame chat with the players about how this will lead to something cool once the effort is put in). Consider the incredible difficulty encountered by real world counterpart codes like WWII’s ENIGMA machine or a book cipher that will require a particular book to actually decipher it. Sounds like a hook into the next quest or a visit to Candlekeep is in order!

Lost in Translation
Comprehend languages works for understanding the literal meaning, not always the intended so some key information can be still lost in that information

What in blazes is a Darmok?”

(“Darmok and Jalel at Tanaga” from the classic ST:TNG episode “Darmok” is meaningless without background knowledge; there’s a whole branch of study that emerged from trying to interpret the context of ancient texts called Hermeneutics).

Another simple way to confound meaning a bit is to put a piece of text through a couple of translators and then back into your common tongue (for me, it’s English). After the third or fourth translation, there’s a decided increase in noise to sound ratio.

Real world examples included alchemical recipes that were often written in poetical allegories that only make sense when combined with a secret lore (something an expert sage might be privy to or a quest to gain the favor of an ancient order who knows a thing or two). Something as simple as a hand gesture or a slight imperfection that is in fact a key that might invert or unlock the actual meaning of a phrase that isn’t in the words themselves (i.e. the three dots at the end of the page aren’t text but a clue to only read every third word). Consider how vague many books that have been seen as predictions have been. The Book of Revelations and the works of Nostradamus continue to have their purpose debated and have been applied to events since the time they were written.

Pieces of a Puzzle
Maybe the text has been eroded over time, be it chiseled into stone that has worn away or a book that has been defaced and torn into bits. Perhaps the pieces themselves have been scattered into various dungeons or guarded libraries to prevent easy access to them. Depending on how liberal a DM is with the interpretation of Mending, something as simple as water damage or spilled paint might be enough to obscure the text from having meaning, prompting the quest for a duplicate copy or finding the original author.

Magic Makes Problems and Solutions
Perhaps the text is cursed and any who dare read it risk some great harm! (There was a classic Dragon Magazine comic that had a wizard reading a tome, the thought balloon reading along the lines of “This is a Tome of Confusion. To read it ib do luz intelijenz …” as so forth. A hidden message might require special circumstances to be revealed, like only in the light of a full moon or if heated by dragon’s breath or a map only visible during a lunar eclipse. If only an elf of royal blood can read it or those runes can only be read from the Ethereal plane, then the party has some questing to do.

Actions Speak Louder
Eventually you want your players to understand what this piece of text actually means, likely bits of it over time. Depending on the reasons behind why this is important to the PCs (is it a prophecy? the true name of a arch devil? a recipe for a potion of gaseous form?) will shape how quickly and easily you want it revealed.

The key is that there has to be some action available to them to eventually discover the meaning of this text. Otherwise, why include it in the first place beyond a little flavor? Will your solution be as simple as requiring an Intelligence roll to solve or recovering a pair of ruby spectacles owned by the author or as difficult as gathering six pieces of a lost map to find a city hidden beneath the sands? If you as DM are blocking the PCs from an immediate solution, then there must be some alternative solution available to them.

Finally …
There is a whole other position piece on the use of languages, exotic and common. I’ve got opinions and a couple of cases of failed uses of them in world building. Comprehend Languages can reveal a clue or even a whole new mystery as needed. Meanwhile, if it’s just a menu from a lost civilization, maybe just let them use their magical Rosetta stone …