I would be overjoyed if any native speakers of German could fix my lousy grammar in YAAC. Alas, it's been 40 years since I actually studied German (as a foreign language) as a student, and 30 years since I had any opportunity to use it conversationally, so I'm not very good at it.
The single file of all German translations does exist in YAAC. In the source-code distribution, you will find in the YAAC/src/main/resources/org/ka2ddo/yaac directory a file named YAAC.properties, which contains tags to all the UI elements in YAAC and their corresponding English text (I point this out so you can see what context in the UI code each tag is used for). The file YAAC_de.properties contains the same tags and the equivalent German text (or was supposed to :-( ). YAAC_ru.properties was created several years ago by a Russian-speaking volunteer, but I haven't been able to keep it up to date since then, so it's missing several hundred more recent tags, which therefore fall back to the default English translations. There are similar .properties files for each of the plugins for additional text that didn't already exist in the YAAC.properties file and its localized copies. For those who are Java language programmers, YAAC uses the Java runtime's PropertyResourceBundle class (with a custom Control subclass to use UTF-8 character set properties files instead of the very limiting ISO-8859-1 properties files that Java supports by default), and the MessageFormat class for substituting variable text at the correct places in localized sentences.
I haven't yet figured out how to invoke localized copies of the help files, so translating them is of lower priority than fixing the actual UI (since it would help to have correctly-localized UI screens to snapshot for the help).
Thank you for even considering helping with this.
Andrew, KA2DDO
author of YAAC