Hey, in my mind, it depends on which area you want to be involved. Python has definitive advantages over R; however, the reverse is also true. If we consider text-based manipulation of large files, and pipeline development, then Python is a clear winner over R; on the other hand, if we think more about data visualisation and statistics, the clear winner is R. R is also good for creating end-user applications now via R Shiny, but Python has this covered, too.
Fortran has a base in bioinformatics and, in fact, R itself is programmed in C and Fortran. Starting out in bioinformatics, though, I would not recommend to anybody to start with Fortran, purely based on the fact that coding in Fortran is absolutely not necessary to forge a career in bioinformatics.
As you mentioned your interest in single cell RNA-seq analyses, I cannot see past recommending that you start with R.
Edit: to give you an idea of my own career: I branched into bioinformatics from a wet lab and comp science background. JAVA and Visual Basic were my strongest languages. ~90% of what I now do is done in R. If I need to work with large text files, I have enough expertise in BASH / shell scripting such that I never have to use Python.