I did look it up, it’s great and faster than my layout for the big languages – German, French, Spanish and Italian – because the accented letters are on the third and fourth levels directly and not written via dead keys, but it’s harder to write most of the others like the Slavic languages (š, ů, ď, ł, ć, ̦ż, ą), Romanian (ș, ṭ, ă), Hungarian (ő, ű), Catalan ( · ), Azeri (Ə), Portuguese (ã), Turkish (ş, ţ, ğ, ı. İ) and Maltese (Ħ, Ġ). In EurKey each one requires Shift+AltGr+DeadKey+letter and a few are missing, while in my layout most of these require one less key. Of course this is not useful to most people, but I’m happy with it.
I agree, all these preferences (currency, time, date, measurements) shouldn’t need to fit in a determined locale box, but if you customize them it’s possible to run into issues. I believe choosing English (Ireland) might solve your issue? Either that or I have found a way to customize it way back and then forgot.
I guess theming would make a lot of the work, I don’t remember the names but there are a few plasma themes that aim to mimic windows 10 and 11. If not the entire theme, the icons at least would make it way more intuitive.
For the office suite you might want to check out OnlyOffice(or even freeoffice), it probably looks more modern and less jarring than libreoffice for someone coming from recent versions of MS Office.
The start menu also has versions of it more similar to the windows one, but the standard one Is much better, maybe it’s a change she can get used to.
Make sure the desktop is in Folder View, so she can put files and folders on it, and set Dolphin’s view to the one she’s used to, icons or details.
I’m not sure for gaming, but maybe check beforehand if hers work on kubuntu.
Finally I’d make a post-it explaining the names of programs: dolphin - > file explorer, Kate - > notepad, ark - > 7zip and so on. The most frustrating thing is looking for a program that should have an obvious name and finding only unknown nicknames, it takes a while to get used to!