I think I know what the problem is. I’ll fix it next week.
Your site’s default locale is set to “English (United States)” and you have customized text content.
In the meantime please make sure that you override the one and other keys or remove the override of the other key.
On your screenshot it doesn’t look like there’s a translation override. I guess you switched the locale in your user profile to something other than “English (United States)”. Customize → Text Content always uses your language, not the default locale. You need to set it to “English (United States)” before you see the translation overrides that your forum uses.
Out of curiosity: Why are you using English as default locale and override some texts with localized content? Why not simply use Thai and customize those texts and contribute missing translations on Transifex?