"\n" is the way to go:
Displays here (Godot 2.1.4) a text with a line feed.
When initializing the text with content from translation csv-files (no auto-init here I think)
I have a hex 0A in the csv-file for translation. (As done by open calc which I use as csv-editor).
When you want to set the linefeed directly in the godot editor (without gdscript) then just click the little icon right of the bbcode value field. A multiline editor will open where you can simply enter line feeds by pressing return.