A personal observation with, I think, universal application:
After getting a fairly solid grasp on my third language, Hindi, I started to realize that there was a fundamental difference between the way my brain processes the languages that I know (English/Spanish/Hindi) and the ones I do not.
The languages I do know seem like, well, languages; which is to say that I cannot help but find meaning in those words, that it feels as if the meaning were inherent. When I do try to hear my languages as an outsider would (that is, sounds divorced from meaning), I'm shocked at what I take for granted. For example, have you ever stopped to think how caveman-like the word "food" is? It sounds like primitive grunting.
Foreign languages, however, don't seem like languages at all, but rather like codes. Like substitution ciphers, they don't seem to exist in their own right, but rather they merely encrypt the languages I already know. In other words, even though all languages employ arbitrary sounds to describe ideas, the sounds that my languages employ feel more "right." I don't think I'm alone in this experience, though, from the way I've seen people learn vocabulary in a new language:
asdflkj = dog
eoriur = cat
poiuwer = tree
Here, the mystery language on the left is being "deciphered" on the right. Notice that when we interpret the equal sign, we don't make a distinction between the idea of "dog," which is beyond words, and the word "dog" itself, which is peculiar to English. Thus, when most of us see such vocabulary lists, we do not see two equally valid arbitrary sounds for a concept, but rather one arbitrary sound (on the left) and one concept (on the right).
This language vs. code idea also shows up in the way we parody foreign languages. For example:
Italian: Montebello, mama mia deliciosso, wella comma to my pizza shoppa!
German: Gutten heiten schitinen hassengurten.
Chinese: Ping pong dong hai ni ma ni pay
I think that people who don't know these languages will agree with me that if these parodies are read in the right voice, they sound close enough to the actual languages. People who do know these languages will, of course, disagree—which brings me to my next point: it's almost impossible to parody a language that one knows fluently. I suspect this is because we cannot separate sound from meaning in our own language as we do with others, so the parodies come off as gibberish.
For now I'm on the fence about whether this "linguistic ethnocentrism" is pernicious or just a harmless natural instinct. Whatever the case may be, though, for the foreign-language learner, this scheme offers some useful heuristics for judging success: if you can no longer parody the language you're learning, if the words in your new language feel "right" and inherently meaningful, you're probably becoming fluent.