Atto has no mechanism for creating a span of right to left running text in a left to right running paragraph or vice versa. You would need this if you were quoting another language inline or referencing the title of a book published in another language.

TinyMCE does this properly.

1. Enable the rtl button by adding rtl to the toolbar config list in Site Admin > Plugins > Text Editors > Atto HTML Editor > Atto Toolbar Settings
2. Write some text in an Atto editor
3. Highlight part of a sentence
4. Click the RTL button

Expect: The highlighted text should be wrapped in a span with dir=rtl
Returns: Paragraph tag of the entire sentence is set to dir=rtl