This is for the Datepicker.js:
In German the translation for "next" and "previous" is depending on the gender of the subject. So it is "vorheriges Jahr", but "vorheriger Monat" (for "previous year" and "previous month"). It is not possible to configure the output to achieve this as there is only one configuration for "previous". I tried to cheat around this by setting "previous" to "vorherige" and "year" to "s Jahr" but this did result in a tooltip of "vorherige s Jahr".