Language settings in a Template Suite determine the language in which templates in the Template Suite will be composed. When setting up a new Template Suite, in addition to entering a Name, and selecting a Country / Region, you will find that Language setting dropdown field. It is important to note Language settings are completely independent of the Country / Region selection.
Template Suites can be assigned have multiple languages, more information about this can be found in the Template Translator article. The language assigned to a template will determine the format of Date and Money Elements.
In the rare situations where users need to change the primary language of a template suite, they can choose from any of the languages that have been published for that Template Suite. In these instances, the Language setting can be found next to the Country / Region setting, in the settings area on the right of the Template Suite.