I noticed this problem
before and it should be fixed in version 1.1.0. Works well for the Polish language.
Delete the sigil.ini file and check again.
Starts Sigil, the language should switch to Thai.
And now in the Preferences you do not have UI Language as Thai?
Thai system language (th_TH) should correctly save in the configuration as:
In my case, the Polish locale language in Windows (pl_PL) is saved as: