Webtrees handles english months - but not local names - while creating dates.
The Hacks for converting month names are version specific but is inserted into includes/Webtree.js
Appended to function function valid_date(v)
:
s=s.replace(/(JANUAR)/,"JAN");
s=s.replace(/(FEBRUAR)/,"FEB");
s=s.replace(/(MARTS)/,"MAR");
/*s=s.replace(/(APRIL)/,"APR"); // Same as in English */
s=s.replace(/(MAJ)/,"MAY");
s=s.replace(/(JUNI)/,"JUN");
s=s.replace(/(JULI)/,"JUL");
/*s=s.replace(/(AUGUST)/,"AUG");
s=s.replace(/(SEPTEMBER)/,"SEP"); // Same as in English */
s=s.replace(/(OKTOBER)/,"OCT");
/*s=s.replace(/(NOVEMBER)/,"NOV"); // Same as in English
s=s.replace(/(DECEMBER)/,"DEC"); // Same as in English */
s=s.replace(/(OKT)/,"OCT");
/*2018-10-14/EBP: Danish months >>> */
datestr=datestr.replace(/(JANUAR)/,"JAN");
datestr=datestr.replace(/(FEBRUAR)/,"FEB");
datestr=datestr.replace(/(MARTS)/,"MAR");
datestr=datestr.replace(/(MAJ)/,"MAY");
datestr=datestr.replace(/(JUNI)/,"JUN");
datestr=datestr.replace(/(JULI)/,"JUL");
datestr=datestr.replace(/(OKTOBER)/,"OCT");
datestr=datestr.replace(/(OKT)/,"OCT");
/*<<< 2018-10-14/EBP: Danish months */
/* Remove . after month */
datestr=datestr.replace(/(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)\.?/,"$1");