Install the UKeyboard package -
http://maemo.org/downloads/product/Maemo5/ukeyboard/
if it already has a Bangla keyboard layout it should also automatically install Bangla fonts.

Otherwise, if there is no Bangla keyboard layout, then it is actually fairly straightforward to create one for UKeyboard The instructions are here http://talk.maemo.org/showpost.php?p=490484&postcount=4

You should be able to use any Unicode compatible Bangla fonts by creating a .fonts directory [mkdir ~/.fonts] and placing them there.

After this you should be able to type Bangla in any application that accepts text input

If there are any Bangla dictionaries available in StarDict format then you can use MStarDict to display that dictionary on your N900. http://maemo.org/packages/view/mstardict/

The same procedure should work for any Indic script