Is YAML with the language code as the root node supported?

Yes, this format is supported. If you have one YAML file per language and each file uses the language code as the root node (as shown below), you can manage them in BabelEdit by creating a Ruby on Rails – YAML project.

en-US:
    dateranges:
      custom: Custom
      last_7d: Last 7 days
      last_28d: Last 28 days
      last_365d: Last 365 days