Are "If I could" clauses always followed by "would"?

I always see

If I could ... I would.

For example,

If I could speak English, I would go to English-speaking countries.

Is there a rule against a conditional clause which contains could, being followed by another could in the main clause?

For example,

If I could speak German, I could understand this essay.

I am a native American speaker but I do not know if the above is actually wrong.

"If I could then I would" and "if I could then I could" are both valid but have slightly different meanings. "If I could then I would" means if I were able to do something the I would do it. "If I could then I could" is just redundant. It means that if I were able to do something then I would be able to do it- not that I necessarily would do it.

They mean different things.

If I could speak English, I could go to New York

If I could speak English, I would go to New York.

In all three cases, could continues to mean

used to express possibility, especially slight or uncertain possibility:

whereas would means

used to refer to future time from the point of view of the past:

Hence, the first means that the trip is possible, and the second means the trip is certain -- both in event of the conditional.