Tooltips use the TextLayout class to render text while all other components use the normal text rendering classes found in Juce.
The TextLayout class is capable of rendering all languages on Windows/Mac/iOS, both Simple and Complex. The normal text rendering classes can only render “Simple Text” Languages, however that actually covers most languages.
Chinese is considered a “Simple Text” language so it should display fine using the usual Juce controls. I think the reason you are seeing boxes is because the font you are using doesn’t have Chinese characters in it.
The TextLayout class also has automatic font fall back so even if you chose a font with no Chinese characters in it, behind the scenes it would automatically fallback to Arial Unicode MS and render the correct Chinese characters using that.