Jacek Caban (@jacek) commented about dlls/mshtml/tests/htmldoc.c:
if(!editmode) test_readyState(NULL); readystate_set_interactive = (load_state != LD_INTERACTIVE);
/* w10pro64_ja has it set to zero despite readyState being interactive, for whatever reason */
if(!is_mhtml || is_lang_english())
The difference is most likely not related to the language, so this check doesn't make sense.