[blawa]

As a computer science graduate, and having worked as a s/w engineer, researcher and quant and currently writing code in strongly typed language, I can assure you that unless you're doing research and writing your own PL, you wouldn't need to understand PL theory, so much so that now I'm rusty in less than 6 years.

Also, just to clarify- PL itself has not much to do Grammars and lexers. PL theory mostly deals with programming paradigms, and type-safety. Lexers and grammars knowledge can help you write a compiler for a PL, but PL theory doesn't care about it- its the job of compiler writer.

I suspect you mean 'theory/models of computation' when you refer to Languages theory (http://en.wikipedia.org/wiki/Introduction_to_Automata_Theory...) That's what covers "language"/grammars. While it's good information to have- a ground up understanding of "computation" science- you would need that pretty late into your foray if you really want to learn about "Computer Science"

CS is a big field- you'll have to choose what you want to learn.

So start with Algorithms first, when you come to the Big(O) notation, you'll get a brief and usually enough introduction to the Theory of Computation.