Although it depends on person to person but I will suggest you go with Java because Java is a more powerful language than Python and has been evolving regularly. Java has maintained itself with the changing times and that means it will be able to constantly provide you more efficient code writing ability. Another reason it has become more important because- As a Selenium automation testers, your growth should be taking you towards becoming Automation framework architects. One of the skills required for being an automation framework architect is the ability to write code. More power of the language will definitely help.
If you want to have a certification in Selenium then you can go with Intellipaat Selenium WebDriver Course.
To learn Selenium free of cost you can watch the following Selenium video tutorial free of cost link is down below:-