"very fluent in" is correct and usable in written English.
You can use it to indicate that someone speaks a language very well. For example, "She is very fluent in Spanish."
He was very fluent in Chinese.
"Four-year-olds, these days, will be very fluent in their use of the app," Ms. Shapiro said.
Some, very fluent in English, were handicapped by cultural differences that led to painful misunderstandings on both sides.
"He's very educated, very fluent in what the people want," said Mr. Jones, a college student in Brooklyn.
But I was amazed to find that he was very fluent in, of all things, Irish Gaelic.
At other times he indicated to her that he was "very fluent in anything computer – there was nothing he could not do on a computer".
Not everything deserves its place but, longueurs aside, this is a very fluent book in which Harries reconstructs not just the man but a time of extraordinary cultural change in which he played such a leading part.
"It was like learning a new language," she said, and it was one in which she rapidly became very fluent.
d. contain very fluent lines.
Its tip is soft and is very fluent.
They speak very fluent Hokkien.
