In Polish there's no difference between good morning and good day; both are dzień dobry (good day literally). Other greetings you can use are dobry wieczór (good evening) and dobranoc (good night), though the latter is something you'd say when you're leaving late at night as opposed to a welcome greeting like the former two.
I would incline against, Jazyk. I have never heard it used like that. Anyway, could you please give some examples you found? Perhaps, the wording is used in other contexts or it is a regional usage that I am not familiar with.