Hello,
When you consider where English speakers come from they're largely going to fall into the following categories: UN and foreign government workers(embassy and aid staff, including Peace Corps), NGO staff (CARE International, Catholic World Relief, etc), missionaries and English-speaking Africans, such as the Nigerian community. There are very few English-speakers, other than neighboring Africans, who are in Niger purely for commercial/business reasons.
For the most part the vast majority of English-speakers are in Niamey and the American Rec Center at the U.S. Embassy is a good place to meet some on a Saturday afternoon. The next major center of English-speakers might be in Maradi as there's a confluence of NGO and missionary workers there. Galmi has several dozen missionaries at their hospital, while nearby Birnin Konni has Peace Corpers and a Nigerian community of traders, plus a few NGO satellite offices. Zinder also has a similar mix of English speakers.Â
After these cities, I believe the next rung of regional capitals: Dosso, Diffa, Tillabery and Agadez would have just a smattering of English-speaking expats. Perhaps others know of additional communities where English-speakers can be found in significant numbers.