Trying to import the German language XML for XenForo 2.2.15 as a new language results in
This most likely happens because language XML < 2.3.0 don't contain short date formats.
Workaround
Manually create a new language and import the XML in overwrite mode afterwards
Suggested Fix
A better error messaging telling the admin that the XML file is incompatible
or
Import without setting short date formats if Skip import checks is enabled (Preferred)