English Translation
Fluency in English is a must.
堪能 is pronounced たんのう in this context
The Japanese sentence is an unnatural robotic word-for-word translation. There are two flaws.
First, 堪能な isn’t usually followed by 英語力. ○○さんは英語が堪能です is a natural collocation, though.
Second, particle は in this sentence suggests that you are “not required anything but English skills.” That’s not implied from the English translation. So, は should be replaced with が.
My alternative translations are:
高い英語力が求められます。(or 高度な英語力)
上級レベルの英語力が必須となります。