Spectrum WebAPP can't display Thai fonts
Release : 21.2
Component : spcint
Spectrum officially supported Japanese locale and the same is mentioned in the docs
If it work for the OC console and the same doesn't work for WebApp then it could be a browser / webSwing config problem.
However, you can try to apply the instructions of the following article:
https://knowledge.broadcom.com/external/article?articleId=250571
Even If it works or not, It's not officially supported.