I personally like a lil klingon flavor in my text and have some in my missions, but , you have to be careful if you do. A lot won't understand it, and most probably don't care. If you decide to put some in, keep it to the common stuff... the occasional Qapla' and PetaQ work well. I've thrown in more but I'll put a translation in parenthesis next to them.
Well, I had the thought of doing all text in Klingon, but then to put the English translation as orange Out of Character text.
Hmm.. this somehow reminds me of this one enterprise episode where they try to fool a klingon ship out of sensor range that they are klingons and where worried that the univeral translator might mess up. It didnt and they had a fluid english dialouge which ended with Qapla'
Just saying, my english is my klingon when playing a klingon.
Personally, I cant see the need to expend effort in making a large chunk or even all the text Klingonese, then put english as out-of-mission text. In fact, i would say that doing this will remind the player that he or she actually speaks english (or other earth language), and is not really a klingon who is fluent in Klingonese, leading to immersion-break.
Although, id have to play and read such a thing to be sure :p
I'll just say, even though i'll be one of the hardcore who'd try and read and take in all the klingonese text, im sure most people would ignore it entirely.
Another thing to remeber with klingon mission is you have more races to play with with very little backstory, one of the best things about KDF mission making is
how much more freedom you have, and that means less downtime repeating common Starfleet phrases like "make it so" and "energize" to suspend disbelief.
I love the much looser power structure and how much more independant the individual Captain can be without elaborate backstory and gimmics.
I would suggest putting in as much Klingon as you damn well please, die hard Klinks will love it, and the rest will just be happy not grinding repeated missions from six months ago!