Which theme are you using?
Locales(Translations) keys are set by the theme developer aren't really standardized between different themes and even the same theme can be customized to have different keys than the original. So giving you the "correct" values is problematic without knowing the theme or seeing the website.
After you fix the issue consider contacting the theme developer and request they either add error checking, or reasonable defaults when a translation is missing.
Always backup themes first before making changes
In the themes page of your admin
Method 1: Restore using Older versions feature for admins theme files
Method 2: Copy values from a duplicate of your theme made before changes
Method 3: Hard reset from fresh theme
Were you able to resolve this?
Following the methods I outlined can let you restore some theme file or data in some situtions
Specifically these are your deleted translation keys from a new install of boundless at time of writing: