So while at first you think "eww, err != nil everywhere", at least in a decade of reading your own or someone elses' code you'll know exactly what's going on.
How many languages are similar? I feel like a lot of languages in the past decade have had a steady migration in things like error handling, so 10 year old Java code is incomparable to today's Java. Whether that's a good thing (the language has evolved and is now more ergonomic) or a bad thing (I no longer know what's going on here) is the big debate.
Learning a language only starts with knowing the formal spec. Then you have to learn how to express yourself efficiently and in mechanical sympathy with the language. That takes time. Worse yet, a significant portion of developers can't do it without a lot of help.
People tend to always exaggerate how little time it takes to learn a language to a meaningful degree. Sure, the first day I tried Go I was able to accomplish something useful, but it took me a few months to develop a basic understanding of how you use Go in an idiomatic manner. I'd say it took at least a couple of years before I could say I "knew" Go.
And it wasn't exactly love at first sight. My initial impression of Go was that it was a bit too much like C, and, coming from Java, I was a bit confused about how you structure things. That takes time to figure out for any language. What sold me on Go eventually was that for my uses (writing multi-protocol server applications that run on multiple architectures), it resulted in code that was very readable and productivity more than doubled because Go is a lot less fussy to work in than Java. Both in terms of encouraging more minimal designs and having a lot less fragility.