No, it doesn't really hold up. To use an example of where I recently had to change the way I thought of things: you might have, say, an intuitive understanding of the logic to make a series of dependent network requests, any of which could fail, but you'll have to reevaluate that when you try a functional language with monadic futures.
My only experience learning a new language that use a completly different paradigm is Prolog. The syntax and builtins make it hard to write something else that what it is meant for.
Do you mean learning or mastering a language. It is kind of different. Looking for "ideomatic python" will help getting pythonic Python code done.
Not going against the spirit of the language helps.
Well, even just using a language (without mastering it) can be a lot more difficult than simply learning the syntax.
For example, many years ago, before I really got functional programming, I tried to learn ML and while I figured the syntax out very quickly, solving problems in a functional style was a lot more difficult and I found it hard to use the language. Learning to use Python or Ruby or Java or C#, albeit not idiomatically, is a hell of a lot easier.
Of course, knowing a language is a lot more than just learning enough to use it, though you certainly don't have to master a language to know it (and many people will never completely master their languages, despite knowing them quite well and using them effectively - for example, I'd wager few people have truly mastered C++).