(First post on the forum!)
I wanted to quickly respond to this thread and confirm that I am experiencing the same issue. I’ve created a forum for people in Japan and the main language is expected to be Japanese. When you create a topic and try to input Tag names in the New Topic UI, the input gets interrupted exactly as the original posted described. I have tested this across Mac, Windows, Linux with Safari, Chrome and Firefox, and the behavior is consistent.
Details:
The way it is supposed to work is that you type the roman letters K→O which would be transformed into the japanese letter KO, but instead the input box interrupts the input and cuts in after every key press, so you end up with K, O instead of K+O = KO（こ）. I suspect this is because the input box is trying to make a call back to the forum service to search for a tag after each key press.
This pretty much makes the Tag functionality useless for users in Japan, and I suspect this is a major issue for anyone (including myself) trying to use Discourse in Japanese.
I have recorded a video of the behavior but cannot upload files yet as my account is too new.