brought up in #2

brought up in #2 ; the $get-language-string callback now reveives a parameter with the current language code.

instead of the hard coded namespace prefix

and update functions accordingly

where some calls to `wega-util-shared:guess-mimeType-from-suffix()` with parameter 'xml' returned the empty sequence

@when-iso, @to-iso, @from-iso
Inputelement $date jetzt element()? erlaubt

when `$lease` is empty, it will not trigger the switch case at
`xs:dayTimeDuration`.

reported by @solveigschreiter

and renamed the module prefix to `my-cache` to avoid some problems with
eXist-db’s cache module which seems to be hard coded

in parallel with app-shared:if-exists()