I've been involved in several projects that hinged upon this question (generally start-ups or web+mobile apps), and it's not a clean or easy answer unfortunately! Plus every developer you talk to is going to try to sell you their services, but what you need is simply the truth!
I'd suggest that you consider shifting your focus away from finding a best language. That sort of premise can eliminate options that are actually quite valid solutions. Keep in mind that any mobile app will require multiple languages working in harmony - all while used in a commercially consistent and standards-based manner. That's the part that determines your technical scalability!
Almost every programming language can achieve this functionality with an experienced developer on your team.
The best advice I can give you, is to consult with at least two people on this, and three if your app is os-specific.
The most important insight will come from a "full stack" developer. One who's got a variety of enterprise experience, and can code at all levels of the stack. This person needs to have experience in leading a team of other developers, which forces the strategy of which technologies to use and why onto their plates daily.
Secondly, you'll want to consult with a "front end" developer, who can tell you what's possible using advanced OOP JavaScript techniques (like Google's angular.js), because in an absurdly fast change over the past 3 years, much of the formerly back-end work has made it's way to the front-end, and is driven by JavaScript, predominantly JSON data, and awesome API's. The game has changed and the front-enders are the poor souls dealing with this rapid shift daily. To do this, they're also fantastic JavaScript programmers, which is a language that runs on all mobile devices and all browsers, too. Love it or hate it JavaScript is the most commonly used language in the world.
Finally, if your app is OS-specific, you'll want to consult with a developer who works predominantly with the OS your app is built for. This person lets you know what's possible from the device POV, should know what stacks and JavaScript approaches can and can't be done on that OS, and how to leverage the resources of the device for your app as well as extend it's functionality.
BONUS - loop back to the full stack developer to double-check the claims of the front-ender and the mobile developers. Always double check with your most senior programmer ;)
All of these consults together in addition to your own research and due diligence will get you comfortable and allow you to navigate on this rather daunting but deeply important journey.
There aren't any turn-key options. Instead it's a series of inter-connected modules driven by different languages, and all working in tandem. Every solution will have bugs, and no one group of technologies can do everything without proper developers.
If you'd like to go further down the rabbit hole, then we should definitely set-up some time to speak. Otherwise, I wish you great luck in research and encourage you to learn as much as you can! :)
It's going to seem hard, and might give you a headache here or there, but learn everything you can about how different technologies "talk" to each other, and then you will be able to build a map for keeping your app and business scalable regardless of the changing tides of technology!