override default pluralize for model-name in rails3

First of all, you need to stop using .pluralize. It uses the Inflector (which is mainly used for Rails internals, e.g. guessing table names for model Sheet -> sheets).

Sheet.model_name.human # => "Beleg"
"Beleg".pluralize # => "Belegs"

What you should do is to use the :count option.

Sheet.model_name.human(:count => 2) # => "Belege"

This requires that you have modified your de.yml as such:

de:

  ...

  activerecord:

    ...

    models:
      sheet:
        one: Beleg
        other: Belege

You can override pluralizations this way:

In config/initializers/inflections.rb

do:

ActiveSupport::Inflector.inflections do |inflect|
  inflect.irregular 'Beleg', 'Belege'
end