"I find having highly-developed English very important skill "

It sounds unnatural to use "highly-developed" to describe one's proficiency in English. Animals such as dolphins and bats can have a "highly-developed" sense of hearing; a country can have a highly-developed economy and so on. "Developed" describes something that has adapted, changed or improved over time and becomes more advanced or effective. 

We can use "proficiency" instead to describe our ability to use a language fluently and accurately.

Some suggestions:

"I find having good English proficiency very important." (Being proficient in English is in itself a skill; thus, I eliminated "skill" at the end of the sentence.)

or

"I find being proficient in English very important."

or

"I find having an excellent command of English very important."

I hope this helps.