Over about 25 or maybe even 30 years I have had to learn many new languages. So many now I've lost count. Here is a rough list.
IF I go way, way back...
8080, Z80, 6502, 8086, 80286 assembler.
BASIC (about 6 varieties).
FORTRAN 4 and 77
COBOL (believe it or not)
PASCAL (several varieties including Delphi)
OCCAM2 (heaps of fun - instant process creation)
C (lots of compilers, lots of processors, lots of strange variants)
Ada (big systems and also embedded systems)
A little Perl just for fun
These have been used on all sorts of machines - little embedded systems, development boards, IBM mainframes running TSO, VAX/VMS, CP/M, DOS, Windows, Solaris, Linux, embedded systems with no OS, remote hosted development environments, on and on the list goes.
And there are a few more obscure things thrown in along the way that I can't remember (or don't want to... PL/M).
Each of these had to be learned because it was part of getting a job done. In most cases I had no experience beforehand, just a tolerant manager and a willingness to leap in and get the job done.
If learning something new is of interest to you, then buy a book, have a play, do things in your own time. If going for a position it looks good on a resume that you have done this because you are interested.
As one of the other answers pointed out, most of the time the concepts for doing mainstream stuff don't change a lot, only the means of expressing it. (Or course you can get into some very unusual languages that are no OO or procedural and this does require a shift in mindset.)
If you get the chance to do something a bit out of the ordinary as part of your job, then grab the chance while you can. Most of those things listed above I have not used for years and am not likely to use again, but you never know. One day I'd really like to build a microcoded APL machine :)