Meowth was trying to impress a female Meowth that he fell in love with, to make them stand out out of all of the other Meowth. (Meowths?) He failed to do so, and now he's stuck with a whole viewer's vocabulary in his lil' brain. xD
Also, the real reason they did this was to make them understand what Meowth is saying. Meowth creates most of the plots for Team Rocket and it would be pretty damn useless if the viewers didn't understand the language of Meowth. And also Meowth's ability to speak the viewer's language helps translate what other Pokemon are saying, which was also another reason Meowth knows the language.
Hope I helped. :)
PS. The reason I'm saying the 'viewer's language' is that Meowth can speak Japanese too ;)
Source: Common Sense and Another Question I don't know the link to.