Support locale aware date formats

**Is your feature request related to a problem? Please describe.**
Entering reading dates currently has to have the format `[day].[month].[year]`, that is, the separator has to be a dot. This is only reasonable for german speaking countrys (?).

**Describe the solution you'd like**
The minimum feature set should be to accept also other separators. But actually the dates should be entered as the user is used to it.