If you change the default language to something other then :en you will see "translation missing" notice.

Example:
p = Product.where(:category => 'good').all
=> translation missing: sl.mongoid.inspection.criteria

This is NOT a bug actually but it is confusing for the developer. I checked the source code and found some translations in mongoid/lib/config/locales/en.yml. Adding the file into your rails app and translating it will work.

Do we have a gem for that maybe?