https://fosdem.org/

FOSDEM is a free event that offers open source communities a place to meet, share ideas and collaborate. It is renowned for being highly developer-oriented and brings together 5000+ geeks from all over the world.» Jyhem has attended for 9 years nearly in a row and confirms that it's huge, it's chaotic, it's fun, and well worth attending.

Most of everything is in English.