No one wants it that realistic, it would be boring as all crap.
As we saw in episodes like ``Vox Sola'' and ``Silent Enemy'', where the difficulty of communication with the alien entities dragged the plots down without giving us any sense of suspense or discovery or alien-ness or ... wait.
I'll grant that too much stuff about Learning To Speak Their Language would get redundant, but then, Enterprise
chose to set up the universe such that most of the aliens Enterprise could expect to meet would be ones the Vulcans had already met and, presumably, knew how to speak with, so that Sato's purpose would be mostly just making sure the system was running right. Of course, they could also have set up the universe so that they did a couple Species of the Week-type stories with the same species, so that a couple bits of establishing How To Speak could be used for a month's worth of stories.
They could also have had conversations with Weekians go through Sato for the first scene or so, and let the audience take their guesses for how long this required to work out communications. This would give Sato's actor more screentime, certainly, although it would pad the story out some in the early parts with a lot of ``Glarble nord flip-flap nasblurtum'' ``He says `hi' and probably didn't mean that about your nose, Captain.''
Anyway, the problem needed more thought when the show was set up: the easier communication is, the less primitive-and-early the adventures feel; but the harder communication is, the more every alien meeting has to be about communication. Declaring that communications is hard except that Sato presses the ``Universal Translator ON'' button leaves everyone unsatisfied.