Restrict editor user to only edit contents in one specific language
The answered Specified here would help you give-permission-to-translate-a-specific-language
After installing Translation Management Module,

You can create users with their translation capabilities
You can specify that a user can only translate from English to Spanish and another user can translate from English to French while creating the user


Create a new user with user role: Translator (which is created when tmgt module is installed)

Then Choose his/her Translation skills as shown below :