You can certainly know multiple languages, but if you are involved deeper in the runtime and language design there is specific investment. I myself did a lot of

You can certainly know multiple languages, but if you are involved deeper in the runtime and language design there is specific investment. I myself did a lot of PHP, went deep into the engine, nowadays do more with other languages where I can transfer some knowledge, but I guess even 5 years after writing my last relevant amounts of PHP I could jump in a consulting gig easily, while for other environments I have to spend more time preparing.