Pronunciation research for app
You need the right audio features, and the modern way to do this is with deep neural networks, which learn them automatically. I would use one built for speech recognition (it's basically the same problem), then model the users by the output of the top layers. Accents should manifest as clusters of the features, and languages as superclusters. I don't have much experience here but I'm making an educated guess.

Here is a relevant paper (Deep Neural Networks for Acoustic Modeling in Speech Recognition), and here is a relevant student project (Accent Recognition with Neural Network).