Learn English – word or phrase for knowing what to say but not how to say it

You are right that ignorant/ignorance often carries a negative sense ("He's so ignorant!"), but it can also be used neutrally ("Ignorant of what had happened in my absence, I continued as before"). It can certainly be used to express humility ("Forgive my ignorance, but who is the current president of South Korea?").

For the neutral sense there is also unaware (and unawareness).

There are not many emphatically positive expressions for lack of knowledge, since this is usually considered a negative (though you might argue innocent has this sense; also ignorant and unaware are often combined with blissfully, after the proverb Ignorance is bliss). But there are expressions for situations where lack of knowledge is expected, and therefore perhaps less negative — for instance, novice, apprentice, learner, beginner, and if your aim is to express humility then any of these would be appropriate.

Consider:

1. Every man to his taste: Prov. Everyone likes something different, and you should not condemn people because of what they like. (Can be used to remark that someone's tastes are different from yours, as in the first example.)
2. There's no accounting for taste: Prov. You cannot blame different people because they like different things, even if you do not understand why they like what they like.
3. Tastes differ: Prov. Different people like different things.
4. One man's meat is another man's poison: Prov. Something that one person likes may be distasteful to someone else.
5. To each his own / Each to his own: Every person is entitled to his or her personal preferences and tastes.

There's also a related (but crude) phrase, "Opinions are like assholes. Everyone has one.", which is well known.