This setting applies to an individual campaign, and overrides your account-wide language settings.
How to change a campaign's language
The admin (owner) of the subscription controls language settings.
To change your language, open your campaign's settings by clicking the "Settings" button at the bottom left of your menu bar when inside a campaign.
Under the "General" tab, click the drop-down that says "Language". Select your preferred language from the list, and click "Save".
That's it! The campaign dashboard will automatically reload, and your new language settings will instantly take effect. These settings apply to everything inside the campaign, and for all users who view the campaign, its dashboards, and its reports.
Please note that this feature is only available on certain plans.