name is wrong. should be theme.po and theme.mo
If your language code is th_TH and it's like that in language table as well, then these should be in path:
oc-content/themes/epsilon/languages/th_TH/theme.po
oc-content/themes/epsilon/languages/th_TH/theme.mo
Make sure .mo one is updated!
Make sure that th_TH is correct code, not that you would like to have such