We do not for example say that the person has a perfect knowledge of some language L similar to English but still different from it. What we say is that the child or foreigner has a 'partial knowledge of English' or is 'on his or her way' towards acquiring knowledge of English, and if they reach this goal, they will then know English.

Knowledge of Language: Its Nature, Origin, and Use (ed. Greenwood Publishing Group, 1986) - ISBN: 9780275900250