Is it a good idea to use many programming languages and technologies in the beginning of your career?

I'm a software engineer consultant working with different clients of the consultancy company which hired me. I'm a fullstack javascript using Node.js and React.js for almost 3 years now. Actually, I'm working with a client for a long period mission and they proposed to me a new project (java, spring boot and react) or the mission will stop because the other project finished. I haven't used java before and I don't have any experience with it and I accepted the proposition in order to learn new programming language. Sometimes, I feel worried that I will loose my experience in javascript especially in Node.js,if I will change the job or I apply for new job because of variety of technologies that I have in cv and I don't concentrated in one thing for a decent period of 5 years or more (that's what employers want when they hire "years of experience" in a specific programming language or technology).

The other reson of accepting it is due to covid-19 crisis and companies don't hire right now and it's risky to change job.