I don't know if this is a well-known methods, but I was able to use save file of Stardew Valley US version in EU version by doing these (and so far I haven't encountered any problems):
- make sure that the save folder of EU version already exists in /user/home/XXXXXXXX/savedata/CUSA06829. If it isn't there, then start the game first (until it reaches main menu and then close it).
- copy both save files (.bin and the corresponding main save file) from the US version's save folder to the EU version's (overwrite).
- copy the /system_data/savedata/XXXXXXXX/db/user/savedata.db file to PC and edit the column `free_blocks` in row/record of CUSA06829 to match exactly like the US version's (CUSA06840) (maybe if there are other columns that are differ in EU and US, you may have to edit it to make it the same as US)
- copy back the newly edited savedata.db file to /system_data/savedata/XXXXXXXX/db/user/ (overwrite).
- then start the EU game.
For the record, the EU version is 1.20 and the US version is 1.04 (or 1.03 I forgot). Maybe it works because the structure (or whatever) of the save file of Stardew Valley between regions are the same, I don't know if it will work in other games.