Star Trek

Why wouldn't the Enterprise have someone on board who could speak Klingon even if they were assigned for the specific mission of escorting Klingons?

Plus one would think Uhura would be familiar with the language being a communications expert.

I know the scene was just there for laughs but come on Starfleet

1) Spock volunteered the Enterprise for the unplanned mission without consulting Kirk. Kirk did not want to go. Time was of the essence because Gorkon's olive branch towards the Federation could have been taken away if opposing factions within the Klingon Empire were able to seize control. I'm not sure they had time to secure a Klingon language expert.

2) The Enterprise couldn't use the universal translator or else the Klingon security post would know the ship wasn't a general supply freighter as was claimed.

The real reason is because, despite how much fans seem to blindly love the movie, Star Trek VI is a fairly sloppy script. I'm not saying I don't like it...but it is a very rushed and hole-riddled film to say the least. 

The fact that a Starfleet Captain with no diplomats aboard his ship is sent to escort the leader of your most feared enemy to the most significant event in galactic history is absolutey ridiculous.

...and besides, the Klingon's weren't given their own language until Star Trek; The Motion Picture.

Uhura just didn't have time to learn the new language!

LOL

Wasn't Spock an ambassador by then?

I think it was more of a "take your son to work day" and Sarek told Spock to run with it as a sort of on the job training, but Spock was just a Starfleet Captain at that time and nothing more.