You never ever rely on just one language, but you must be proficient in multiple languages for a successful career.
Being from automation background were I have worked with both Java and C# in combo with Selenium, so I try to give you a better idea.
When you start with Selenium I would say JAVA would be a better option for many reasons:
There are a lot of opportunities for Selenium with Java is higher in the market as there are very fewer opportunities in C# with Selenium.
By searching with the keywords like ‘Selenium with Java’ and ‘Selenium with C#’ in any job portal you can verify then things.
For Java, there is large support available online, as most of the professionals are using Selenium with Java.
Rank wise Java is among the top 3 most popular languages, C# is not.
On the other hand for C# if you stuck at any point then it would be quite difficult to get help from the online community.
If you want to start learning Selenium then you must choose the right resource, you must sign up for industry-based Selenium online courses. For your reference, I am mentioning a video tutorial for you. .