How does one keep ourself good at both when work demands sometimes Python and sometimes Java

In my career, I had started with Java. Then, worked on project which had more of Python and at times on Java. In next project it, required python, and no java. Then, next one requires Java and some python. Next one, I don't know it may require Python or Java or both. How does one keep ourself good in a programming language which one has worked on in the past but not working on now?  What I understand is that a language on which we worked on in the past, but not working on currently may become rusty and we may not know the latest features and may not get constant practice. So what are the ways to be good in at language which we worked on in the past but not working on currently ?.

I think many programmers would have experienced this situation and their comments will be valuable for me.

Thanks.

Only 2? Over the years, I've worked with Fortran, COBOL, the assembly languages for various platforms, LISP, FORTH, Smalltalk, Perl, Rexx, Javascript and Python. Just off the top of my head. Oh wait, also Pascal, Modula-2 and Ada (for college course work). I really should take a serious look at Kotlin, but there's no immediate compelling need. Hmmm. Forgot Java, ironically.

I've also had to be fluent in IBM mainframe Job Control Language (JCL), the linkage editor, and their equivalents on other systems. Plus scripting for several different OS command shells, such as Bash.

And Domain-Specific Languages (DSLs) such as YACC/Bison, Lexx, PCCTS/Antlr, Ant, Maven, Gradle, and so forth.

I could stretch the definition even further if you include utilities like RCS, CVS, Mercurial, Subversion and git.

In short, it's a very rare time when I've only used a single "language" even on a single project.

So, in short, you just accept that you cannot be at peak in everything all the time. If you can be competent, that's all that really matters. You'll tend to catch up as you get back into a particular groove and tap into up-to-date reference materials. I used to also get a lot of help from paper tech magazines, but those have gone by the wayside, alas. Nothing like idly paging through a magazine to make discoveries that you might have otherwise missed.

So, in short, you just accept that you cannot be at peak in everything all the time. If you can be competent, that's all that really matters. You'll tend to catch up as you get back into a particular groove and tap into up-to-date reference materials. I used to also get a lot of help from paper tech magazines, but those have gone by the wayside, alas. Nothing like idly paging through a magazine to make discoveries that you might have otherwise missed.

Thanks for sharing this and the advice.
I have a question. If one has worked in a particular language for many years but not working on it in current project, then to still be competent,  what are some things he can do ? Is devoting time on weekends on it the only way?