Thank you for getting back to me.
Unfortunately I am unable to use the same third-party tool for the Vietnamese keyboard.
Note that the Track Changes functionality does not depend on the specific language. The typed characters just appear in pure HTML formatted syntax.
For example, when the tracking is enabled, the typed text is getting in <ins> tags to underline it. I suggest you trying if the keyboard used is supported with such underline formatting.
Additionally, as I mentioned the MS out-of-the-box Vietnamese keyboard is working as expected, and I can only recommend using this one to ensure that it is supported.
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