I wouldn't know how to do it via the 'scryfall deck' command, but you can do it using 'scryfall print lang:[language] [cardname]' where [language] is the two character abbreviation for the language code. [cardname] can be in English or the target language.

These are the only language codes that I would assume work:
en, es, fr, de, it, pt, ja, ko, ru, 汉语, 漢語 (surprisingly, even the latter two work perfectly, at least if you copy-paste them)

For example, "scryfall print set:bng lang:漢語 ephara god" will give Ephara, God of the Polis from Born of the Gods in whatever language that is (can't read it).

Two important notes:
1. Be warned that the artwork may be really low quality, which is no doubt because of a bad scan on Scryfall's part.
2. The hover text for the card will still be in English. There's nothing that could be done about that short of messing with the code.

idk if this is the intended behavior, but at least it seems to be possible in v1.966.

change language of "scryfall deck"
1. go to scripting to importer
2. overwrite variables near the top of lang = 'en' to lang = '[language code]'
3. save&play

ｌooking at the script roughly, I found that when fetching scryfall's API, the global (?) variable lang is used if the json's lang is other than 'en'. I thought it might be, so I changed it and it worked fine.
Please let me know if there is a correct way.

P.S.
I noticed MTG Deck Loader have language options...
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2163084841