One theme that I use for my WP website has .po files for language. I trying to download to my localhost .po file to edit it(translate it) and upload back to host. The problem is that when I upload that file again nothing change on the site. Like I didn't tranlsated it. I download and upload both .po and .mo files.
I use PoEdit and I have no idea why is this.
Did you try to rename them on actual language? I mena if you translate en_EN.po and en_EN.mo to let's say French when you upload them you must change their name like fr_FR.po and fr_FR.mo. Or whatever language you will use after translation.