Honestly, C# and Java are vaguely similar (GC languages, lots of OOP, with some FP sprinkled in, like LINQ/Streams) and some of the more popular frameworks, such as ASP.NET and Spring Boot shouldn't be too hard to pick up - so personally I'd go for both.
COBOL or FORTRAN or older languages might be okay if you want to specifically take advantage of that niche, but won't make you quite as hireable everywhere else.