hi, sorry, i'm not a native English speaker, but i think with trial and error i found a temporary solution so i'll share with the class.
1. i downloaded the file that was shared before in this thread, Solved: Re: The Sims Medieval issue with downloading - Answer HQ (ea.com)
2. i closed the EA App and uninstalled it [maybe unnecessary, but better safe than sorry] and using an instruction from /r/origin i installed Origin
3. i started downloading the game, at first without the expansion pack [maybe unnecessary, but better safe than sorry]. the downloaded game had 5.19 GB too, and it began downloading again after reaching 5.19, but not from the beginning, but from about 3.7 GB. then, without pausing the downloading, i put the downloaded file (point 1) to the folder. C:\Program Files (x86)\Origin Games\The Sims Medieval\GameData\Shared\Packages - is the location in my case, in yours it may be slightly different at the beginning if you chose different location. there was a file with the same name in the folder, but it was about 500 MB [and the new one is over 1 GB]. Windows asked if i want to swap the files, i confirmed. at that time there was a notification on origin about some error, i clicked okay and the downloading paused. after the new file was in the folder, i clicked to continue downloading. then its status changed to "finishing" (or sth like that). after a short while it installed the game, in my native language as i wanted (so there is no problem with language, cause i saw there may be). then you can download the expansion.
unfortunately my files and saves got lost when origin transformed into ea app, but at least i can play the game from my teenage years again ^^
extra point 4, i didn't try to do it cause i didn't feel like it
4. you can install the ea app again [if you uninstalled in in point 2] if you still need it [from what i've gathered, on origin there are problems with the friends list, achievements etc)