Location, but needn't be too localized. (I think most people travel nationwide)

I'd filter on language, but also include "concepts" (for instance, mobile, or startups). You also should identify conferences which aren't immediately obvious what the language is, or that cover other languages. (for instance, RIACon was traditionally a ColdFusion conference, but in recent years has added a JavaScript track; Ruby conferences usually have some functional talks, etc)

Honestly, I'd initially just filter by region or continent. Going down to the country region only adds complexity for people who don't mind traveling to countries nearby. And the volume of conferences isn't SO high that you'll get a huge list by selecting just a continent.

Or maybe "technology". For instance I'm only interested in speaking at iOS/app conferences. I speak on design, programming, and business. So, a single language isn't as important to me.

Technology, not location. Farther-away conferences are a plus if you enjoy travel!

However, I'm never going to be an expert in PHP...