Seriously, what does it mean (I'm confused as a newbie)?
It's used, often comically, to present a favoritism bordering (or long past) the level of religious belief and fanaticism towards the superiority of a specific language.
For example, "Python is the One True Language" would roughly mean "I strongly prefer Python for most situations and I don't mind showing a bit of fanaticism about it". When applied to another, it could be used in the previous sense or derogatorily: "for him, Python is the One True Language" roughly meaning "even when it's not the right tool for the job at hand, he'll try to force a solution in Python".