A bit late with a solution/workaround but hopefully this helps someone.

TortoiseSVN seems to be determining the Windows locale in a wrong way. It seems to use the language used for the Windows date, time and number formats. When I changed the formatting language to "English (United states)" the problem was fixed for me.

If the spell check you want to use is English but you want your date to be formatted the German way you could change the formatting language to "English (German)".