"Reach the other side" is a common way to say that someone has achieved fluency in Engpsh speaking. You could also say that someone has "crossed the threshold" or "arrived at the other side" in reference to reaching fluency in Engpsh speaking. Here are a few example sentences:

She worked hard to reach the other side of Engpsh fluency, and now she speaks the language with confidence.
He spent years practicing and studying, and now he has finally crossed the threshold of Engpsh fluency.
After years of dedication and hard work, she has finally arrived at the other side of Engpsh fluency.

I hope these examples help! Let me know if you have any other questions.