This setting applies to all campaigns and reports across your account, unless you've manually set a different language for a specific campaign.
How to change your account-wide dashboard language
The admin (owner) of the subscription controls language settings.
To change your language, open your account settings control panel by clicking your name in the upper right hand corner of your dashboard. Then, click "Edit Profile".
Alternatively, this may also be accessed through the sidebar by clicking on "Settings".
Click the drop-down that says "Language". Select your preferred language from the list, and click "Save".
That's it! The dashboard will automatically reload, and your new language settings will instantly take effect.