Despite understanding one or several languages, most people don't even understand what a language is. They think it consists of specific sounds, words, visual symbols, and the arrangements thereof. This is actually a large part of why it's so difficult for us to learn new languages after a certain age - we become highly proficient at picking up the pieces of a language while missing its substance. Allow me to demonstrate.
Let me state as a disclaimer that I am only monolingual, so any grammatical corruption, awkwardness, or horrifying gibberish that occurs in the course of demonstrating this point is purely the result of colossal ignorance, and nothing untoward like a glitch in Google Translate. Look at each of the following passages - without putting them through a translation program or trying to decipher them - and decide what language you think they are.
ゴ アヱ ユ ボ ゾ
哪里 是 在 钱 谋杀绿脚趾
это является не русский
You might guess the first is Japanese, the second Chinese, and the third Russian - or you might suspect it's a trick and say that they're meaningless gibberish, but that's not correct either:
All three are English, more or less. While it's true there is no one-to-one correspondence between characters, sounds, and words in one language and another, in fact there is no such correspondence between one person's usage of the characters, sounds, and words in the
same language and those of another, so in fact we reach a profound understanding: You can say things in English using symbols other than the Latin alphabet, and in words other than those that appear in the English dictionary, and the same goes for all other human (not logic) languages. Sounds, visual characters, and words are arbitrary - what defines a human language is the relationship between the components.
In the above examples, the first one puts together a series of Japanese Katakana syllables and says "go a we (pronounced 'way') yu bo zo." In other words, you're using a Japanese character set to build a phonetically English sentence. It conveys no meaning in Japanese because it's constructed on representing the sound of spoken English rather than specific objects and ideas, but it could just as easily have meaning to English speakers if they knew what each syllabic character represented. In other words, it is English, just as much as the alphabetically abbreviated text-speak people send over phones and post in Twitter.
The second and third ones are constructed by feeding individual words one at a time into Google Translate. Go ahead and feed the second one into Google Translate and turn it back to English...:-). The third one is just "This is not Russian." Again, it's not a real translation - it's just a construction by putting together Russian words one at a time into a more or less English sentence. And basically that means it is English - not perfectly, of course, since that would require a mastery of both languages to approach, and because as noted it's not really possible to say that anything is perfectly equivalent to anything else in or between human languages.
This understanding about the interchangeability of parts in human language - phonetic, visual, and relational - is in fact the basis of cryptography: Sentences in a human language are passed through vastly complex logic language filters to come out the other side as something that is neither immediately decipherable to natural speakers of that language nor itself logical. And yet even though it looks like gibberish, or is even completely unrecognizable as a language - e.g., steganography - it remains as much English as the arbitrary arrangements of visual patterns you see on your screen right now, or whatever other language originated the message.
So when you hear people for whom English is a second language say things like...
"It rain today."
"I am to the store going."
And so on, they're not speaking English incorrectly - they're not speaking English at all! They're speaking their own language using English words. And unless you're fluent in some other language, you probably do the same when you try to speak another. And yet you can understand them and they you despite arranging things differently, pronouncing syllables differently, etc.
Sometimes people can even understand each other with no mutual commonalities in language, and this leads to an even more profound realization: There is no such thing as English or any other human language - they are all fluid associations of experiential cues (e.g., sound, visual patterns, objects, relations) that have been arbitrarily grouped by random variations over time. They all flow into and out of each other all the time, arising from, reflecting, and in turn shaping our evolution. So when you run into redneck assholes who have a problem with other languages being spoken around them, tell them you know of a restaurant they might like: