From time to time it happens that the wrong language is selected when creating a SharePoint site collection. According to Microsoft there is no supported way to change the language of an existing site collection. This is the case because the provisioning language impacts the names of the pre-created items such as document libraries. From a user perspective to most annoying problem is the wrong language of mail notifications.
If you only need to ensure the correct mail language there is an easy workaround. Open SQL Management Studio and execute this query on the respective content database.
Update [dbo].[AllWebs] set Language = [LCID]
This will change the language on all site collections but you can add where-statements to update only the necessary entries. The LCID is the language code used by Microsoft (and SharePoint).