Within the WAR-archive* you find the directory
"WEB-INF/classes/de/lotk/yawebmail/properties/"
where several language-files are located (e.g. "view_en.properties" or
"view_de.properties"). The two letters previous to the dot conform to the
ISO-language-code
(List of ISO-language-codes).
Make a copy of any language-file into the target-file
"view_#language-code#.properties" into the same directory and do the
translations.
If you don't want to wait, you can add the language-file yourself (in this case please mail the translation anyway, so others can take advantage of your work, too):