Go with Python definetly. It's used everywhere by web developers for backend developments : API, website backend, workers... but also by data scientists (lot lot of resources, models and libraries in Python it's language #1). For the web parts, best web framework are in Python : https://stackshare.io/microframeworks (Flask #2 and Django #3). Java is good but trend is not great in terms of popularity amongs developers and tech leaders.