As of OS 10.6 the Mac doesn't fully support Bengali. Matter of fact, detour to Character Viewer tells me that 10.6 didn't even come with Bengali fonts (they would be in the Optional package, which was not optional in my case ;)). The ones I've got are Alphabetum and Code2000, both third party.
But full rendering support is the bigger issue. As you know-- or I assume you know, since Sinhala may not look like Devanagari but it's genetically related --there are quirks involving consonant clusters, particuarly with -r-, and also the special nature of short "i". To say nothing of the vowels themselves. (Further detour to Character Viewer turns up a bunch of dotted circles with "kombu" in the name. I thought a kombu was a kind of soup, but I guess I was mistaken.) Cursory googling suggests that support was added in .7 or .8.
How did you get interested in such a wide range of languages?
I'm a font junkie. That's all. And a language junkie.
Do sing out if you ever trip across a utility that converts Mac OSX keyboard layouts into a format Windows can use. I can make Mac keyboards standing on my head, but the Windows folks tend to be left out in the cold. (Linux users can generally take care of themselves. That's a description, not an opinion.)