Is OK to giving away a key for Sword and Fairy 4? (A Simplified Chinese Only Game)

It's not against the rules. I am not really privy to the demographics of the site as to how many fluent Chinese speakers there are, but it's ultimately on the winner to look at the game info (and request -though you're not forced to grant it- for a reroll if they join by accident).

Do note there are both bots and people whose sole intent is grabbing +1s for the library, so if you want a more selective audience crank up the minimum level and consider specific groups or regional restrictions.

How useful or sarcastic answer do you want?

First of all, as long as the key works, the site itself doesn't care about the language, all good according to rules.

You can make it region locked to China, so you can sleep knowing it gets to somebody who can at least read it. But you're locking out everyone, who can speak the language, but doesn't live in China.

You can make a public giveaway, where every single bot, autojoiner and not-caring user will join it, and likely that it won't get played for years, or ever. Yes, even if you write the description that it's Chinese only, because most users don't read the description because the aforementioned automation, language differences, or for lack of care.

Another option - make a forum giveaway for it, either just behind some simple anti-bot shenanigan. Or - if you're speaking the language (translators should work well too, I assume), I'd make some easy puzzle in Chinese, so one needs to know the language to get to the game - and a reminder during the puzzle, that the game is currently Chinese only. (Some puzzle fans would translate and solve it just for the sake of it :D )

You can make giveaway set to only China region if it will help you feel like game went to someone who would understand it. But chance game will be actually played anyway is like 0,1%, cause literally tiny amount of sg users care for what they are even entering.

Since there are Chinese people living all over the world, I don't see the point of going to any lengths when creating your giveaway.

Just note in the description that it's Chinese-only and request that only those who will play it enter. Not a perfect solution but it will help somewhat.

Machine translation is a thing even Inuktitut became recently available on Google Translate.

Any giveaway you make for any game will get entries, not only because there are bots, but also because people enter to win games for all kinds of reasons. I remember considering entering for a game that had nice a pointshop background (then changed my mind, I prefer spending my SG points on things I want to play). If you want the game to be played, it's safer to give it away in a group that requires you to play your wins.

Make invite only, make a puzzle, say that it is chinese only. You might be surprised, someone might actually play it. You can also advertise a white list recruitment for this game only and ask that the winner be willing to play the game to improve odds further. But up to you how you want to go about it.

As Gilgamash said, put it in a post in the Puzzle/Events forum, behind a jigidi (takes little to make one at jigidi.com. Make the post in chinese with only one english sentence (this game is in chinese only), and ofc the giveaway invite only. Only those really interested will enter the giveaway.

If you only care about breaking the rules, there are no rules about what kind of game you can make a giveaway for, as long as the system accepts it as an existing game/dlc/whatever. So just do a giveaway and let anyone enter it, but as others said, also you'll get many bots and autojoiners.