I do not think that Wittgenstein is saying that if a lion could talk, we could not come to learn and speak his language. I dare say that the possibility of a language being learnable is a necessary condition of actually being a language. At present, lions other animals, and babies make noises, but there is no regularity in use that gives their noises a meaning, thus we do not say that their noises are words, or part of a language, they are merely noises.
In order to discern what Wittgenstein is saying with his lion aphorism, we need to set the context. The comment comes in Part 2, section xi of the Investigations, where Wittgenstein talks about seeing an aspect and experiencing the meaning of a word. Seeing an aspect, is like seeing an apparently unfamiliar face and then, all of a sudden, recognising it as the face of an old friend; it the thing that happens when we see the figure below as a duck or seeing it as a rabbit:
Wittgenstein then discusses the possibility that somebody might be "aspect-blind", that he lacks the ability to see something as something. Now clearly, this is not the same thing as being actually blind, he would still be able to see the figure above, he would simply not see it as a duck or a rabbit.
Wittgenstein then moves on to talk about "experiencing the meaning of a word". This is the sort of thing most of us do when we read poetry, or when we look through synonyms in a thesaurus for "the right word". The analogous phenomenon to aspect blindness, here, is meaning-blindness. A meaning-blind person would not understand the difference in what "I will show you fear in a handful of dust" and "I will scare you with a fistful of dust" mean to us, the sort of meaning that poets manage to conjure up with words. He would not understand if we ask him to say "party" as a verb, and then the next moment as a noun? He simply says party, he is incapable of saying it as anything. Yet, as in the case of the aspect-blind person still being able to literally see, the meaning-blind person will still be able to know what words mean the sense that he will be able to use the language. This is because, for Wittgenstein, meaning is use, and one doesn't need to have a particular experience, or indeed any experience, to grasp a sentence.
In the case of the lion, it is preceded by this passage: "
We also say of some people that they are transparent to us. It is, however, important as regards this observation that one human being can be a complete enigma to another. We learn this when we come into a strange country with entirely strange traditions; and, what is more, even given a mastery of the country's language. We do not understand the people. (And not because of not knowing what they are saying to themselves.) We cannot find our feet with them. " Wittgenstein is using "understand" not in the sense of being unable to understand his language, but being unable to understand lions and their practices. If lions could talk we would be able to understand their words, but that does not mean they would be anything like the characters in The Lion King; imagine having a conversation with a lion in its natural habitat, it would not be wise or proud, it would just be weird.