Input validation checks are such a small part of the codebase; it feels weird that it would dictate the choice of a server-side programming language. Server-side python is very capable of checking the length of a string, for example.
One challenge is that you've got to keep the server-side and client-side validations in sync, so if you'd like to increase the max length of an input, all the checks need to be updated. Ideally, you'd have a single source of truth that both front-end and back-ends are built from. That's easier if they use the same language, but it's not a requirement. You'll also probably want to deploy new back-end code and front-end code at the same time, so just using JS for both sides doesn't magically fix the synchronization concerns.
One idea is to write a spec for your input, then all your input validation can compare the actual input against the spec. Stuff like JSON schema can help here if you want to write your own. Or even older: XML schemas. Both front-end and back-end would use the same spec, so the languages you pick would no longer matter. The typical things you'd want to check (length, allowed characters, regex, options, etc.) should work well as a spec.
It's also not the only place this type of duplication is seen: you'll often have input validation checks run both in the server-side code and as database constraint checks. Django solves that issue with models, for example. This can be quite efficient: if I have a in my HTML and I want to add an option, I can add the option to my Django model and the server-side rendered HTML will now have the new option (via Django's select widget). No model synchronization needed.
As others mention, you may want to write additional validations for the client-side or for the server-side, as the sorts of things you should validate at either end can be different. Those can be written in whichever language you've chosen as you're only going to write those in one place.