If you want to do a job properly the language you use only makes up a portion of the overall picture. Other skills are also important, things like a robust testing regime, version control, consistent style using best practices, security, etc. When you learn to do these things correctly in one language they are easily transferrable to another, the effort put in will not be wasted.

While I do all my backend work in Perl I also do a fair bit of frontend work, which means I have to use Javascript. I have never really had to 'learn' Javascript as I have mostly been able to write it in a similar style to the way I write Perl, and will look up the odd thing on a case by case basis.

I also do some Wordpress work which means using PHP, and the switch to that is even easier. Everyone looks down on PHP but Wordpress powers 65% of CMSs today and there is tons of work.

As for Python, the few times I have had to deal with it didn't go too well - the change in mindset was painful and slowed me down significantly. I'm sure others will disagree, but I don't think the transition between Python and the other languages mentioned is as seamless as that between Perl. So for me, Python is the outlier and if I were to focus on another language I would probably choose Javascript.