From what I understand of your question, you are not hiding coded messages in the runes/inscriptions/text, but merely want them to have to hire someone to translate instead of using the spell.
As the rules are written you have little recourse.
The spell however gives you some options.
It doesn't clearly explain what touching means. Do they have to run their fingers over each letter/word while under the effect of the text? This is a good way out of your dilema.
Many ancient civilisations could very well trap their inscribed walls/objects. The movie the Mummy has a particularly nasty acid trap in one of the sarcophagi.
If they have to touch every word then have thousands of words. They will require multiple castings. Most PCs only memorise 1 utility spell at a time.
The following suggestions are Rule 0 territory in general ...
At leat with priests you have another option - does their deity actually care about diviniation spells? If not the deity may reject their prayers for Comprehend Languages.
As for Wizards, don't allow them to access certain spells during the game based on where they research their new spells. In my world, for example, divination of ancient texts has actually been systematically restricted throughout history (because certain people/races don't want the truth to come out about a past age).
Bards fall into the Wizard category. However their forte is understanding the unknwon. So perhaps there is a special guild out there (hidden of course) where they might find what they seek.
Another option which I really like is to read Sean Reynolds' webpage. He has an article regarding absolute spells versus skill modifier spells.
The gist of it is that some spells work automatically when they really shouldn't. For example Find Traps gives you a skill bonus to Search. Comprehend Languages should give a skill bonus to Decipher Script as opposed to automatic success.
The article's full text can be found at:
http://www.seankreynolds.com/rpgfiles/misc/variantfewerabsolutes1.html
and
http://www.seankreynolds.com/rpgfiles/misc/variantfewerabsolutes2.html
I think that this is a much better solution, elegant, and doesn't penalise a player for wanting the spell. You then set the DC of deciphering high enough so that it requires a lucky roll, or a sage of some description.
Of course a bard or rogue with appropriate access (spell knowledge or use magic device) could get past your DC but it makes it a little harder.
D