> However, if I try to do the same with Devanagari script (itrans inscript), table columns don't get aligned properly.

Use [`valign`](https://elpa.gnu.org/packages/valign.html) from GNU ELPA.

From the blurb

> This package provides visual alignment for Org Mode, Markdown and table.el tables on GUI Emacs. It can properly align tables containing variable-pitch font, CJK characters and images. Meanwhile, the text-based alignment generated by Org mode (or Markdown mode) is left untouched.


Looking at discussion in [`valign` issue#27: Alignment doesn't work for org table with indic scripts
](https://github.com/casouri/valign/issues/27), I bet that it works with *any* indic script.

If  you haven't already noticed ... *You do NOT need to do all the jugglery with fonts and their widths suggested in the other responses.*  This is because the `valign` library measures pixel-width when doing alignment.