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How is it received by the readers to see the dedication in, say, Chinese, while the entire other parts of the thesis or a book is in English?
Would that annoy the readers who do not know the language of the dedication? Would that be considered as "putting private message in a publicly-designated document"?
Is there a reference that says whether this is a good/bad practice?
You're allowed to write a dedication in whatever language you'd like. Most readers skip the dedication page anyways, so it's not as if it's going to really influence their view. I'd find it interesting, myself, but others might be slightly confused to see a different language or alphabet. But I doubt many people would stop reading just because they saw something like that.
Supervisors will not make you change your acknowledgements or dedication. This section is entirely up to you. It is your personal choice what to include here and your choice alone. My supervisor did not even copyedit mine, he marked it clearly as such so that I was aware that I had to proofread it carefully myself.
You can acknowledge whoever and whatever is important in your life, I've seen PhD theses dedicated to God, deceased relatives, and even Cup Ramen. It's not uncommon for these to take an informal tone and have in-jokes directed at particular lab members, friends, and family how provided support.
There are no recommendations here. However, it is important that your readers (and especially your examiners) can read and understand some of it. Specifically, they will check whether your supervisor(s) have been acknowledged. If they aren't it can indicate that there were problems between the candidate and the supervisors (or that the thesis was completed and submitted without their oversight). It depends on the examination process of your institution but your relationship with your supervisor and whether they supported submission of the final version could be looked into.
Otherwise, anything goes and it depends who you wish for your acknowledgements and dedication to be directed at. If English is not your native language, it is completely acceptable to include a small passage as long as the research content of your thesis can be understood by your examiners (usually in English). It is your choice whether you translate this.
For example, here is the ending of my acknowledgements section for my PhD thesis. This is directed at my partner, also a scientist, who is Japanese (we were doing long distance at the time). No one has ever questioned me on the meaning of this or why I included it.
You can find the full thesis here for an example:
