For your clarification, to apply RTL to the paragraph you have to use bidi property. If you change alignment property alone, it behaves like normal English word. You can check the same behavior in MS Word also.
To apply RTL to the paragraph, you can use below two options:
1. In paragraph dialog, use Right to Left direction options.
2. Use bidi property in selection paragraph format.
documenteditor.selection.paragraphFormat.bidi = true;
It will right to left direction to selected paragraph.
Please let me know if you have any questions.