To achieve this you will need a script to call the tr() function on the translation keyword.
One way to do this would be to attach a script that defines an exported string property that is then run through the translation function and assigned to the text field.
This is a C# example, but you should be able to convert it to GDScript pretty easily:
public class TranslatedRichTextLabel : RichTextLabel {
[Export]
public string TrKey; //enter your translation key in the editor
public override void _Ready() {
BbcodeText = Tr(TrKey);
}
}
For simplicity you may want to write a separate script for a standard label, but you could also detect the node type to determine which text property should be assigned to.