If I select an Arabic font like Kufi (كوفي) and enter text into GIMP 2.8.14 on OS X 10.11, this is the result:
As you can see, the letters are incorrectly disjointed.
If I right-click on the text and select "From right to left", it makes no difference. In the text tool's options, I can change the language from English to Arabic, but it makes no difference. I can also change the justification to centered or to left-aligned, but it makes no difference.
Some fonts I select do render with the letters connected, but it looks like it's simply using a fallback font.