Time- and Dateformats used inconsistent

[ramack]

Currently we have a setting for the default_datetime_format, which changes the view in the history to be user-adjustable. All other date and time formats are fixed and only selectable by the translators.

We have to

• [ ] decide which formats shall be configurable by the user (do we need it at all?)
• [ ] clarify in the translation files which format strings are used only for formatting (I. e. the letters shall not be translated, but the format adjusted to the locale) and which are visible in the UI and should be translated (currently none, until the next point is solved)
• [ ] for configurable datetime formats do we want to have the formatting string translated? (Y is J in German or HH is ЧЧ in Russian for the user input, while the formatting still uses "HH" as two-digit hour format) - also check whether this is already supported by Android libraries.

[vit1-irk]

If the format string requires explicit user input, in fact, it must not be translated. But if it is about to be just displayed, it may be translated.