I'm guessing that Ubuntu probably would have Pikolo's grandparents language needs covered. For ease of use, I still think that Ubuntu's Unity is as simple as simple can be right out of the box.
I thought Steam OS was going to be the bridge to better gaming in Linux. This sure is a bummer reading what you and Pikolo have to say about gaming in Linux. I say this because gamers are often on the cutting edge of computing. I don't want to see Windows monopolizing anything, including gaming.
I used to play 2 games: World of Tanks and War Thunder.
WoT released an official tutorial, how to run it with Wine... 2 years ago. I tried following it with the updated components, and failed. I tried with Play on Linux, failed again and gave up. After all, it's games that should seek after players, right?
WT on the other hand is heavily focused on being portable. They support 7 types of hardware, including SteamOS, Steam ran on Linux and a standalone Linux version, except it's not a .deb file, no that would be too much to ask for

It runs on minimal settings, similarly to it's Windows performance(thx Intel HD! No irony here, GJ for making it work so well), but that's just because my laptop isn't meant for games.
If you follow OMG Ubuntu, new games are releasing Linux versions daily, and newer games usually have them right from full release(alpha/beta releases on Linux are something I've yet to see). They usually have 40% lower performance compared to DirectX implementations, and this is especially relevant on machines that have to choose between 30 FPS and medium settings
My grandparents are Polish, and I know the language is available on Ubuntu. They even have a polish portal(
http://ubuntu.pl/), albeit the language on it is very... stiff? formal? artificially sounding for sure. But correct and understandable. Unfortunately, I don't know how well is the system translated.
Some programs are translated better than the others, for example Open Office(haven't bothered to upgrade it to since before the LO fork) has a beautiful translation.
I tried using Peppermint in Polish and gave up quickly. The translation is half-done and leads to a lot of confusion. Translation is something that Linux has catching up to do, or at least so I though. My Vista is 100% in Polish. My grandparents Windows 7 is a mix, and if not for Team Viewer, they would need to seek a local professional. But if Windows continues to degrade, that will make the barrier of entry ever lower.
BTW, if you need help with the translation PCNetSpec, I should be able to help. I'm on the first year cybersecurity, so I should get the basics of OS inner workings by winter
The DE performance doesn't matter that much, though I'd probably try to get them Cinnamon as it looks similar to what they consider a desktop. Unless I'm missing something, the default XFCE layout mirrors the OSX shell