advice to Upgrade skills

You CAN use any .net language that you want of course, but the vast vast majority of SharePoint books, documentation and Code Samples are in C#. The official Microsoft SharePoint SDK Samples is only in C#. https://www.quora.com/What-is-the-best-way-to-learn-SharePoint-development https://www.newhorizons.com/article/the-top-sharepoint-skills-you-need-to-learn-today