Seems like Malte is messing around with some packages :)
Ok, what this errror is saying is that a query is trying to insert an new internationalisation key into the table for message keys. But the system will only accept keys for packages installed. Thus you need to install the package acs-translations which is basically an empty package which can contain any package independant keys.
Please grep for the sql query and tell me where this insert is taking place:
insert into lang_message_keys
(message_key, package_key)
values
('rel_type_contact_rel', 'acs-translations')
Either you take that insert out or the package that does this call needs a dependency to acs-translations.
