Not understand the whole language...?
“A lot of people look at C++ and want to understand every dark corner. This is what you do if you want to be a compiler writer, but most people should look at what is the easiest way to write a good solution for their problem," said Stroustrup.
Oh no. Heck no. Hell no. F*** no.
That's hacking, not programming. It leads to training people to hack, rather than to program.
If I appear to be repeating myself, it's because it's a Very Important Point.
The world has enough different languages already, generating lots of incompatible code. When your language encourages people only to learn subsets of itself, it becomes effectively a myriad of closely related languages, but undefined and uncontrolled languages. "So you're a C++ programmer. Does that mean C++.subset(a,b,c), .subset(d,g,z) or .subset(f,m,dribble)?"
I suppose it may be fairer to call it "dialectisation", but the end result is that code is harder to share and maintain, because programmers' expectations and assumptions vary so far from each other's.
People hack the solution from the coding techniques they know, because the right way to do it is obscured.
If you really want a language that supports multiple subsets of functionality, you have to find a way to segregate and mark them clearly and unambiguously, so that programmers are able to identify what they know and look for what they don't know.