No, they are not all speaking English. According to James Gunn, the writer and director of the Guardians of the Galaxy trilogy, all of the Guardians (except for Groot) have implanted translators. It can be presumed that most of the other GotG characters, like the Ravagers, have these implanted translators too.
These translators are not universal, however, and may fail to translate some languages. For example, in Guardians of the Galaxy Vol. 3, Nebula, Drax, and Mantis encounter a group of children who have been imprisoned by the High Evolutionary for his experiments. Mantis says upon hearing them speak, “That language isn’t in my translator.” This implies that the translators have a limited database of languages that they can recognize and translate. Nebula and Mantis didn’t understand the children, despite them both having an implanted translator. Only Drax knew their language without needing a translator.
Relevant James Gunn tweets (2023-03-26):
Question: James, is it true that all the guardians talk in the language of their planets but manage to understand each other thanks to translators present in their necks?
James Gunn: Yes.
James Gunn: Translators work both ways. All of the Guardians other than Groot have implanted translators. They understand and can be understood by others.