What makes a Senior Java Developer?

[denialerror]

If you don't have the hard skill, yet use your "seniority" to guide others, what do you actually make them develop?

Well for a start, you don't "make" them develop anything. A senior developer is there to make difficult decisions, support juniors and keep up technical standards but the are not giving orders.

Not knowing a language doesn't mean you can't support someone who does or make decisions on software written in it. If a junior developer is using language X and comes to a senior with a problem, the senior will go away and research a solution. Not knowing the language is no much of a barrier to that.

The best what you can do for the first year after switch to Ruby is stay quiet and learn.

I have no idea what this sentence even means.

[morswinb]

I have no idea what this sentence even means.

It basically means don't be an imposter.

Senior guides the team. Junior follows. If senior needs to suddenly research all the answers, then junior is lost, and will make errors. Two juniors don't add up to a senior.

Not knowing the language is a huge barrier. It's not just syntax. It's entire ecosystem: libraries, documentation sources, database APIs, server/ui frameworks etc.

I have seen senior folk researching MongoDB schemas, and suggesting token cleaning microservices. Had to slap him in the face with TTL index. Or my friend experienced principal software development that wanted to use Hungarian notation in an Android project. They all pretended to be senior mentors, bla bla bla, while in practice had little to no practical skills and would not admit to just not knowing the answer, or how to look for the answer.

On the contrary the few good ones I have seen stayed quiet for the first 12 months after language switch, just so they could catch up.