immerse + in/into
Which preposition would you choose?
"Immersion into/in a language is the only real way to learn to speak that language as only a native can."
I'd use "into", because it's rather the target of a movement than just a location.
By the way, the word "only" in your sentence makes it illogical (from a formal viewpoint): a non-native wants to speak a language as only natives can. He can't do that by definition, so I'd omit "only".
I don't know; neither choice really seems unnatural to me there, but I think I would prefer "in". I Googled these phrases, and "immersion in a language" gets 12,300 hits, whereas "immersion into a language" only gets 1,260.
But I agree with Ant_222 that the second "only" should be omitted.
It's from the Latin, in- + mergere = to merge [with a liquid]. I'll vote for "immerse in" since 1) "immerse" implies the object is already in the liquid before it's submerged, and 2) one could claim the "im-" suffix means "in", so saying "immerse into" would be really saying "submerge in into".
I'd reword the sentence as: "Immersion is the only real way to speak a language like a native." That avoids the whole problem altogether. :) That said, "immerse" is usually used with "in", even when motion is implied: "Immerse yourself in Spanish!" rather than "Immerse yourself into Spanish!" But "into" doesn't sound bad in that particular sentence. I'm not entirely sure why.
I don't think the word's etymology is relevant. English is not Latin, and current usage of Latin words is often far from etymologically correct (and often "etymologically correct" usages are incorrect English!).
Generally things are immersed IN, not into. "I was immersed in that book", "Immerse the item in the dyebath for at least 30 minutes", "Immerse yourself in another culture".