I'm assuming you want to change the error messages of the built-in validation methods.
When you use a validation method, you can specify a custom error message on a case by case basis through the optional message param. For instance:
validates_acceptance_of :eula, :message => "must be abided"
Otherwise, you could try editing the ActiveRecord::Errors.default_error_messages hash so changes affect all validations. This is what the hash contains:
>> y ActiveRecord::Errors.default_error_messages
:invalid: is invalid
:wrong_length: is the wrong length (should be %d characters)
:confirmation: doesn't match confirmation
:taken: has already been taken
:accepted: must be accepted
:not_a_number: is not a number
:blank: can't be blank
:inclusion: is not included in the list
:too_long: is too long (maximum is %d characters)
:empty: can't be empty
:exclusion: is reserved
:too_short: is too short (minimum is %d characters)
Try putting German translations (the following courtesy of babelfish) at the end of config/environments.rb:
:not_a_number => 'nicht ist eine zahl',
:exclusion => 'ist reserviert'