Here are some reasons why speaking English is a really great idea:

Language of literature:
English is the language of literature, with many classic and contemporary works of literature being written in English.

Language of law:
English is the language of law, with many international legal documents and agreements being written in English.

Language of finance:
English is the language of finance, with many financial institutions and markets using English as their primary language.

Language of politics:
English is the language of politics, with many international political organizations using English as their official language.

Language of medicine:
English is the language of medicine, with many medical journals and textbooks being written in English.