Flash ZTO over AT&T?

[setorseth]

Hello guys...

this is my first post here, so, please.. .bear with me... =)

Lets go

My mom went to USA few days ago and purchased a second hand AT&T Galaxy S6... so far, so good. But, as long this device came with Lollipop and only a few interface languages to chose (my mom only reads portuguese, yes, I'm brazilian =)... ) I decided to update to Marshmallow.... and now, my quest begins

1- I've been reading a few people where updated their AT&T S6 to Marshmallow, got a not fully functional device (sometimes no carrier, sometimes no wifi, and the worst one, the phone get locked outside US)... does someone can confirm this one? I don't want to throw my mother's money in the bin...

2- As long AT&T firmware does not provide Portuguese interface language, can I flash a ZTO stock firmaware into this device without brick it? The hardware is the same sold by AT&T, so I thing wouldn't be an issue to to that.... or it has?

Sorry to post this kind of question, but I couldn't find any solid answers to my doubts...

Cheers =)