Since I will be able to change in time after that, and the other instructions don’t really work (outdated) – here is a guide on how you (without Terminal) in the language.
The Router has been reset (OpenWrt 15.05).
At the very bottom you’ll find the commands, if someone likes to work.
Returns you to the router menu on your IP and logs you in.
System –> Software
Click on Update lists and wait just a second, then continue below:
german as a search term to enter, and to Find this package , click.
Click on Available packages (English)
And the point luci-i18n-base-de – at install, such as the top of the image.
Confirmed with OK
Languages and Style
Under Languages click on the auto button, and then in the list, Deutsch (German) select.
At the very bottom of the page, click Save & Apply click –> short wait and that would be it.
Who would like to change it via SSH/Terminal, execute these commands:
 opkg update
 opkg install luci-i18n-base-de
Should there be any questions, then just write.