English is by far the most important language on the web. It's as simple as that.
Even if your customers are not native speakers of English, there's a good chance they speak at least a little English. It's not about aiming your business at the British or American, for example, it's about being truly international.
English is simply the lingua franca of the world.
Having your local language and English side by side looks more professional than 50 badly-translated languages.
If there is one language you should offer alongside your native language, choose English.