If you click the tab My organisation - Company details, there is a setting called Country and Language. The following text describes the differens between this settings. 

Country:  The tab Country shows how personal ID numbers are validated on the user account. There are different syntaxes for personal ID numbers in different countries. Remember to also fill in the address details of the organisation. Some countries require full addresses for different categories.

Language: Is preferred language for all the files or e-mails that are generated for or by that company, and default one for users that are invited in that company. So, if you invite some user, they will get an e-mail in language that is defined here. If you as admin send some reports or wants to download generated PDF version of report, you will get in this language.
 

Good to know: This setting only applies to new users. The language on existing users who are invited to an additional organisation will be based on the selected language on the account.