[XeTeX] Bug or design flaw in Polyglossia 2012/04/29 v1.2.1

[Ian-Mathew Hornburg]

Regarding the \today macro, there’s no way to do what you want and
still use that macro without some sort of workaround that you could
code up to affect the way that command works. There’s patching
functionality in the etoolbox and xpatch (extends etoolbox patching)
packages that may help you do what you want. I patch a few core LaTeX
commands for doing a similar kind of font switching in some different
use cases.

Otherwise, you can just manually do a \date{<Malayalam>\textenglish{.}
<Malayalam> etc.} command in your preamble.

Also, you can grab yourself an updated copy (5.0.1) of the Rachana
font (and others!) from the Swathanthra Malayalam Computing folks
here: [http://download.savannah.gnu.org/releases/smc/fonts/].

Hope this helps.