[Q] ATT Unlocked root question topic

I have a Samsung Galaxy S5 I got at AT&T and paid it off... so it's completely mine. Then I had them unlock the phone for me so now I'm in Ukraine using this phone, but it's in English and there is no way I could have the phone be completely in Russian. I've used a third-party app called "MoreLocale" to change the locale to Russia (had me do some ADM/fastboot codes to allow it) but that didn't completely change it to Russian.

It changed some applications to Russian which have Russian translations to it, but the ROM itself is in English... I'm trying to get it to Russian, but I'm afraid to root it and flash a new ROM on to it as I'm afraid I might somehow re-lock it to AT&T service or something else. Basically, I'm afraid of loosing the use of this phone. A phone like this one here in Ukraine is expensive, and they don't have deals like they do in US where you get to pay off monthly.

So my question is, is it safe for me to root the phone and flash a custom ROM onto it?

Another question, if it is safe for me to flash a custom ROM onto it, does it have to be AT&T compatible, or can it be international version now that the phone is unlocked?

Thanks in advance.