Knowing basic programming concepts (data types, function, conditionals, loops, control flow, classes, methods and more) is a must
Language depends, if you want to build native apps: for IOS the latest swift version. for android a language that supports JVM (Java and kotlin are the two most popular so far I know)
you could also build a hybrid app. I found a good article:
The question in your case (something you will need to research) is if the hybrid framework of your choice is capable of reading the sensor.