I did a fresh install of the latest Tumbleweed and during the installation selected US as region, however there are several bookmarks in Firefox pointing to Taiwanese Mozilla localization.
Here are some of the bookmars:
https://www.mozilla.org/zh-TW/firefox/central/
https://support.mozilla.org/zh-TW/products/firefox
Does anyone have an idea as to why those bookmarks are automatically added in Firefox?