Best programming language to develop mobile games with?

Which programming language would you recommend learning in order to be able to create a game like Flappy Bird?

I have front-end skills like HTML and CSS (and a design background), and want to quickly learn a language to start with game development.

Mobile gameS are written in many different languages, depending on the distribution platform. iOS for iPhones has been the most popular in the US, but Android platform is growing faster globally. That said, from web dev front-end skills to programming on those platforms is a pretty big leap, because you have to learn server side development and different environment, not just a language.

What I would suggest for you is to experiment with Single Page Apps. SPAs are basically web applications loaded in a single page native wrapper. You can find mobile dev kits online for that for free to play around with. There has been quite a trend developing mobile applications this way because a single web application can run on the web and various mobile platforms. The language I would suggest for you to learn would be JavaScript, it's still mostly front-end, but you can actually write NodeJS applications that run JS server-side. In general, JavaScript is an excellent skill/language to learn and is in high-demand.