Talk

The national language is Bengali (Bangle) and is spoken everywhere. It's an Indo-Aryan language derived from Prakit, Pali and Sanskrit and written in its own script. Many Bangladeshis understand only limited English such as basic affirmatives, negatives, and some numbers. This is especially so in rural areas and among the lower socio-economic classes. Learning a few Bengali words ahead of your trip will prove very useful.
Two centuries of British colonization lead people to identify most foreigners as either British or Americans, and to view them with curiosity. The first question you will probably be asked is "What is your country?" ("Desh kothay?" in Bangla). If hawkers or rickshaw-wallahs are over-zealous in selling you their products or services, simply say "Amar dorkar nai" ("I don't need [this item]") or "Lagbey nah" ("No need") as a colloquial way of saying "No, thanks."
If you don't wish to give money to beggars and other unfortunates, simply tell them "Maaf koro" (with informal you) or "Maaf koren" (with polite/formal you), which means "Pardon me"; or you can apply a tricky concept by saying "Amar bangthi poisha nai", meaning "I have no change." Above all, if you're refusing a service or product, don't linger. Walk on as you say these phrases. Otherwise, your lingering may be misinterpreted by peddlers as your uncertainty about refusal.