add custom translation file

[sirnone]

add the possibility to overwrite the Admidio translation files with an own file in a directory that is not overwritten on every Update. the translation from this file should be used every time, and if there is no entry in this file, we use the system default.

[Fasse]

A good place could be adm_my_files/languages and there the same filenames like de.xml or en.xml.

[thomas-rcv]

Sounds good for me. This option makes it more customizable

[ximex]

https://www.admidio.org/forum/viewtopic.php?f=20&t=7225