Hello TaigamaZ,
Good to see you in Microsoft Community.
I reproduced the scenario you described: Open a local PDF file with Microsoft Edge browser and click "Add text" to add text in different languages (English, Chinese), but after confirming, the Chinese characters would disappear.
We confirmed this to the Bing Team: PDF viewer supports a few languages right now when adding text. Unsupported languages will disappear. I hope that answers your question.
You may feedback your questions in the Microsoft Edge browser. Open the problematic page in the Microsoft Edge browser, then use the shortcut keys "Alt" + "Shift" + "I" to open the feedback window (Note that the feedback window will capture the current screen and URL link and email address, and you can add Attachment and record your problem), enter your problem, and click "Send".
If anything is unclear, please do not hesitate to let me know.
Best Regards,
Mosken_L - MSFT | Microsoft Community Support Specialist