No, English is not a Celtic language. It is a Germanic language that evolved from the languages spoken by the Anglo-Saxons, who migrated to Britain from present-day Germany and Denmark in the 5th and 6th centuries.
The Celtic languages are a separate branch of the Indo-European language family and include languages such as Welsh, Irish, and Scottish Gaelic. While Celtic tribes inhabited Britain before the Anglo-Saxons invaded, the languages they spoke did not significantly influence the development of English.
Over time, English incorporated a number of loanwords from Celtic languages during its evolution, especially in place names, but the core structure and vocabulary of English are rooted in Germanic languages. Thus, while there are connections between English and the Celtic languages, English itself is classified firmly within the Germanic branch.