tl;dr - this
does work to a point, but won't necessarily teach you idiomatic and community practices that come with experience, but it is surprisingly sticky
I had the great pleasure, year ago in my undergrad Operating Systems class, for the class assignment to be "write an OS in Java"...which of course was handed out to a group of students who had never seen Java. By the end of the semester we had written the core guts of a multi-tasking OS, a couple shells and the display systems to handle even displaying things like a unix-like console, a sane piping system, all the major user land utilities (sans some of the compiler things, but things like ls, cat, ps, etc.) a simple text editor and intra-system messaging system, etc. etc. etc.
It was a great curriculum and really was the first time we, as CS students, had the chance to really spend time understanding the subject matter without spending time focusing on stupid language tricks like we had in our various C and C++. The code we wrote was fairly straight forward (we were learning the language as we went, so kept to the KISS method) and focused instead of the material. It was probably among the hardest, and best class I've ever had on any subject.
Did I know Java at the end of it?
To a point -- I knew the pidgin dialect we wrote the OS in. A few semesters later I took a fluff software engineering course and had to hack out some various java server bits and had a roughshod time of it as I ran head first into the now common overengineeringitis that plagues modern Java development. I found the syntax and most of the standard library familiar, but the idiomatic ways of writing the code, community practices, the shibboleths, nearly impenetrable without years buried in an enterprise software house.
I swore off Java and never looked back...moving on to Perl and Python for a spell (incidentally my standard "learn a new language" project is to write a simple non-lexical phrase extractor, it touches I/O, data structures, database connectivity, program flow, and if I get daring, multi-threading and a few other odds and ends and usually gives me a pretty good idea how a language works.
Now years later, taking a look at Android dev, I'm finding that writing code for the platform, even though it's Java, to be like writing code for our old OS. It's pretty simple, there's great library support, and I don't have to wrap simple method calls in hundreds of lines of framework boilerplate nonsense. It's actually pretty fun.
But I've definitely been drawing heavily on that pidgin dialect of Java that I learned way back when -- it's kinda like riding a bicycle, except a few bits have changed here and there. So yeah, I think I did "learn" the language, and it's been amazing how much of it I can recall since it's been a decade since I did any coding in it.
(this method also handily solves the "I need a project, a goal, to learn the language, otherwise I'm just twiddling bits" problem).