Jira fields have English names, even though my Jira installation is a different language.
All REST calls by Backbone are done via the synchronization user. Backbone enforces this user to use English as language. This way Backbone is able to analyze the error messages returned by the JIRA REST APIs and provide help articles how to resolve these errors.
Unfortunately, this also has the effect that all other information Backbone requests with this user is also in English, e.g. field names.