What level do I need to get to before I can understand a video game in Japanese only?

It is likely going to be difficult no matter your WK level, so why not start dipping your toes in now.

I don’t think your WK level will have a huge bearing on this, as grammar will likely play a huge role, and I expect most games to contain rare and unique vocabulary.

As others have mentioned, it depends on what kind of games you want to play.
I’d +1 the recommendation to play a game with a game script (more on this below) which you can have open on your computer while you play, so you can lookup words in yomichan as you go, and ideally a turn-based game or a game you can easily pause, maybe with furigana.

It also helps if you can find a game that you are interested enough in to warrant the effort it will take to play it, this is the same for other native material.

Have you tried immersing in any other native material?
if not, consider joining the Absolute Beginner Book Club to work on your general reading skills with a group which will help build skills that translate over to reading in Video Games.

Youtube resources

Here are some Youtube channels in this area:

I’d highly recommend watching some videos of these channels playing various games to get an idea of what is out there.

Games I’ve tried

Super Mario odyssey is a great first game

  • furigana.
  • most text is pausable or progresses only when you want.
  • instructions are (generally) repeatable, many include animations to illustrate moves.
  • good use of colours and emoji to add context to the text.
  • Japanese quest videos means you can get help.

Other games:

  • I’m (slowly) playing Demon Slayer - has Furigana + I’m reading the Demon Slayer manga in Japanese, so lots of cross-exposure.
  • I’ve started Final Fantasy IX, which is slow as there is no Furigana but it is a game close to my heart and there are game scripts online when I need them.
  • I’ve Done some Vocab mining from other game scripts including Ace Attorney and Professor Layton, and I’m planning on playing Dragon Quest XI later so I’ve starting dabbling in mining key vocab from that game (by watching Japanese Quest play through).
  • I’ve heard great things about Yokai Watch 4 and Ni No Kuni but I haven’t tried either myself yet (both are waiting on my Switch).

If you have a Nintendo Switch then setting up a Japanese account is pretty easy, and then you can download Japanese game demos from the Switch store - many of them are very generous, for example Dragon Quest XI’s demo is something like 10 hours long IIRC.

Game scripts

Here are some links to lists of Japanese game scripts and a discussion on how to find more:

Finding a game’s difficulty
Jo Mako’s list linked above ranks games by their Japanese difficulty, there’s also this WK thread roughly ranking games by their Japanese difficulty

If you didn’t know, Yomichan (setup instructions) is a browser plugin that can do dictionary lookups for words you hover over, so it is amazing for going over Game Scripts while you are playing a game - rather than typing the word out.