Hello Jutta,
Yes, you are right. The de translations are a long-standing only-second priority problem. And certainly more work could be done. I am glad to see that you both propose it.
As I answered to @Jan_Hoppe, you do not need any specific permissions to extract the de_DE contents from Transifex. (I have no further feedback from him though. His two accounts on Transifex have been aded to the de_DE project.) The recent post on Mongolian translation contains a recipe for a test installation. Together with the Indico installation manual you should succeed to install your own instance.
Assuming that your pseudo on Transifex is “Any One” (Can you change that?), I approved your request to join the de_DE project.
Would you mind to subscribe to the mailing list mentioned above to discuss de-specific details? There are several sites in Germany which already use one version of german translation, which has unfortunately diverged in the past. I expect that this can be settled soon, and it should not prevent you from testing the existing in practice.
I hope to hear from you soon.
Dirk