The sentence is grammatical, but it has a common problem: it's not clearly stated. I had trouble understanding it and had to read it and the question a few times before finally understanding what it wants to say:
If I {couldn't / didn't / weren't able to [CHOOSE ONE]} speak Spanish, I {wouldn't be able to / couldn't [CHOOSE ONE]} speak to my parents.
If you want to emphasize the importance of being bilingual in this case, revise the clauses and focus on your ability to speak Spanish as well as English. Then give the reason why it's important for you to be bilingual.
Word order matters. Which is another way of saying that style matters.
The problem with much of what we say and write is that it's not clear because the syntax is a little screwy, the word choices are less than optimal because the words either aren't precise enough or because they're ambiguous, the word order makes the sentence ambiguous, or the focus is on the wrong part of the sentence.
Yes, it's correct grammatically.
But you usually use "all" immediately after a subject pronoun when there's no auxiliary. For example:
We all worked hard on this.
In case of an auxiliary or two auxiliaries, you usually use it after the first auxiliary. For examples:
We are all working hard on this.
We have all been working hard on this.
Alternatively, you can start your sentence with all of + object pronoun. For example:
All of us have been working hard on this.
The use of "all" after the subject pronoun in the sentence with two auxiliaries presented by the OP is correct grammatically, but it's less common.
Some people use all after the second auxiliary, which is also less common. For example:
We have been all working hard on this.
Best Answer
The word category flat adverbs covers words that can be used adjectivally (to modify a noun) and adverbially (to modify a verb) without changing the form of the word. So, for example,...
...where we1 might have expected quickly, loudly, really slowly.
OP's example probably wouldn't be explicitly identified as a credible "flaqt adverb" by any recognised authorities. The usage does occur, but there are ten times more instances of the "correct" version happened so suddenly, and personally I don't think there's any reason for supposing that using the "incorrect" version would make native speakers think you've got an ear for relaxed colloquial speech patterns.
TL;DR: In essence, it's a mistake. But not a serious one, and it wouldn't normally be explicitly recognised as an error in ordinary speech. Except by pedants (and perhaps TEFL exam setters! :)
1 By "we", I really mean "you" (learners), of course. Native speakers learn how to use flat adverbs by repeated exposure to examples which they then repeat in their own speech. Learners are much more likely to think in terms of applying a "rule" they've been taught (adjective + -ly = adverb).