Varied languages whether actual or fictional in my writing has played a big part for very personal reasons.
Beyond those personal reasons, I have discovered that varied languages within a fictional story, simply by virtue of the characters not being able to understand one another can setup some great moments. A good example of this you may have seen is the movie 'Hell in the Pacific' with Lee Marvin and Toshiro Mifune, wherein a U.S. and Japanese pilot find themselves encountering one another on a deserted island.
My first effort at building a fictional language, began as I was playing a chat-based RPG, my primary character a barbarian. Throughout the many years I played, the culture of that barbarian clan grew and expanded, and along with it the language. Besides adding a richness to the character, the misunderstandings, conflicts in culture and language, all vastly improved play for all a considerable amount. Though my character was the only one who spoke it (along with a pidgin Crown/English), in time many people had picked up enough to converse simply.
Currently what I'm working on, also has a language attached to it. Pastoral Pidgin. Pidgin, slang, expletives and so on all of us have used at one time or another and still do. We've all no doubt when younger even intentionally used it whether to seem 'hip' (yeah, I'm old), try to have some secret code or simply generate a common bond with our peers.
In any case, the most important thing IMO to remember when developing a new language, is to have a very firm grasp of the culture using it.
Ex.: Police have their bits of slang they use... So how would it read if an officer responded to central dispatch; "I can dig-it central dispatch. Everything is groovy here. Car 302 is making like a banana and splits."
So really focus on that. For my barbarians, the number/quantity "three" was considered sacred. So was balance. Too weak or too strong, unfaithful. Naturally there was much more to it, however, by knowing the culture of the people so well (naturally it helping that I devised that culture), I could develop a language that fit their deepest values and mindsets. It truly helped the process considerably.
Nevertheless, if you start there it will come off as much more reasonable than just some random words.
I'll mention a bit more of my experiences (though by no means of quality, simply what I have discovered) when I have more time.
K2