Developing an App for my school

Which language or languages is most suitable for a record-keeping type program that has both a browser interface and a mobile app?

Some context.

My language school – in St Petersburg Russia – recently spent a small fortune on hiring a private developer to create a central network for teachers and administrators to keep track of attendance and what material teachers had been covering. It's a godawful thing – takes forever to load on a website, we can't view classes of other teachers, and its only in Russian language, which isn't suitable for all of us, and other clunky things that make it very user unfriendly.

I'm far from experienced when it comes to programming – I did a bit of python in school and have made some simple tools for myself as a hobby – including a framework for making modding games easier – nothing exciting.

But an app like this doesn't seem to be the most complicated thing in the world to create. Maybe I'm just inexperienced, but I guess the main components are just a database for classes and teacher accounts, and an interface for accessing and modifying it from both mobile and browser.

Any tips on which language would be best to use would be super helpful! It'd be great to have a good reason to get back into coding! Thanks!