Hi Arshia,
As you’ve surmised we don’t currently have support for complex script alphabets. We have plans to integrate a font shaping library in the future, though I can’t give you an exact date.
We actually distribute the shaping library we are likely to integrate with our GitHub source. http://site.icu-project.org/ provides a shaping library we would likely use.
As for where you’d go about doing such an implementation yourself… we would definitely make changes to SEditableText in Slate to perform the shaping automatically, but to make ti available to other use-cases… perhaps FText.Shape()? It would return a new FText with the shaped text (since FText are intended to behave immutably).
I honestly haven’t put much thought into this yet. 