A Splash Entry in Chinese has wrong grammar. The entry was 你好中国!.
This is grammatically wrong in Chinese, the actual text should be 中国你好 or 中國你好 (Traditional).
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https://www.google.com/search?q=%22%E4%BD%A0%E5%A5%BD%E4%B8%AD%E5%9B%BD%22
https://www.google.com/search?q=%22%E4%B8%AD%E5%9B%BD%E4%BD%A0%E5%A5%BD%22
Google returns quite a few results for each (206,000 for the former, 1,900,000 for the latter). Even if it's technically wrong, it's apparently used that way by actual Chinese people. It may actually be intentionally "wrong", as several of the English messages certainly are, as they reference various internet memes.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Confirmed in 13w25c.
And Japanese text '日本ハロー!', I can't understand what it means. '日本の皆さん、こんにちは!' is better, but word 'こんにちは' is used only in daytime.

'ハロー' is "Hello". It's probably supposed to sound kind of funny. It may refer to the "Hello Japan!" TV channel. Many of the splash messages are references to obscure things, and various things that look like mistakes are actually intentional. They're just personal messages by the developers, only of meaning to them and other people in the same circle, who get the joke. Only whoever added any particular splash message could determine whether or not it's "wrong". Though it wouldn't be hard for them to signal their intention, given the presence of this issue on the Mojira... Considering the presence of similar messages in Greek, Russian, and Korean, whoever added them probably just put "Hello, <country>!" in an automated translator.
Whatever is in the splashes is intended, how broken it might seem 😃
I do speak Chinese, in fact, 你好中国 is correct. Translating to "Hello China"
If changed to 中国你好, will output the result as "China Hello", which (usually) makes no sense in Chinese.
Why are you expecting a Chinese translation to make sense in English?
Because it is on the splash screen (which is global as far as I know), this does not belong to the language translation change.