The past perfect indicates an action or state that was completed before another past action or state began.
I had learned English before I entered the university
certainly does NOT mean
I was in the process of learning English before I entered the university.
The past progressive here indicates that the process was incomplete through the time that "I entered the university."
"I had learned" has perfect aspect, which means that the process was complete in SOME sense before "I entered the university."
However, you cannot determine in what sense the process was completed. That depends on context, tone of voice, etc.
I had learned English before I entered the university, but I was not prepared for the nuances that I encountered in my classes in law.
This does not imply perfection. It implies some lesser degree of skill, but it does imply sufficient skill for some (most?) purposes.
English tends not to imply fine distinctions in meaning through tense alone. To imply perfection, say
I had learned to speak English like a native before I entered the university.
EDIT In a comment below, Ronald Sole has pointed out that perfection in English is probably impossible for anybody to achieve; it is almost certain that even Dr. Johnson had an occasional lapse. Mr. Sole also makes gentle fun of my example’s equating native speakers with perfect speakers. That equation is of course wildly contrary to fact, but I was discussing grammar rather than reality.