I've been doing backend development for more than 3 years now using Python as my preferred language and Django as web development framework. Both offer a great combination of simplicity and productivity. You can develop REST APIs for your web or mobile applications quickly, and there are great Platform as a service companies like Python Anywhere that are not expensive (even free if you don't need many resources) and hide the complexity of setting up the environment yourself. Off course, if things get big, you may need to use more flexible services such as AWS.
Either if you need to start from scratch, or if you already have a backend system and need to expose an API for your app, I have tons of experience on the subject and can build it for you.
More recently, services such as Parse or Firebase also offer a good alternative to avoid having to develop your backend from scratch. I've also worked with those. Personally, I prefer and enjoy getting my hands dirty with the backend, but for productivity reasons, sometimes these services are great alternatives.