When importing WKS spreadsheets, cells formatted as date have 'TT.MM.JJJJ' format or similar. Even if used locale has nothing with German.
Created attachment 56998 [details] patch This patch should amend the issue. It also translates comments in relevant file.
Urmas committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=0b8d8ab30305212c3606d9218f1e017132d4772b fdo#44738 use general format code keywords
Eike Rathke committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=cb7ff0d3801868d6ed7ba9be79095cdce9e29915 changes to fdo#44738 patch to make it work with other locales
I had to revert the German translation part due to lack of a clear MPL/LGPLv3+ license from Umas, and no response on that. The fix should stay fixed though.