Culturally English seems to show up in anime and manga a lot. However, for One Piece there is an additional reason. This story takes some inspiration for characters from real life pirates (and other individuals) which were primarily English, Spanish, French, and Portuguese. The characters are generally not Japanese.

Just focusing on Strawhats:

Sanji, Robin, and Brook use french for their attacks. Robin also mixes in Spanish while Brook mixes in German. Franky is obviously an American and keeps speaking American. Luffy, Ussop, Chopper, and Nami seem to be mostly English speaking. Basically, Oda some times borrows words from the languages he thinks are appropriate for that character whenever he wants to for emphasis. Zoro is thoroughly Japanese despite not being from Wano. Instead of languages, he names otherworldly imagry for emphasis.

Interestingly, this doesn't line up well with the nationalities in the SBS. Otherwise, Robin would speak Russian, Luffy would speak Portuguese, Ussop would speak an unspecified African language, and Nami would speak Swedish.

The Doflamingos use Spanish. The World Gov. uses English. This does not explain all uses of English and it does not explain them at all in universe.

Culturally English seems to show up in anime and manga a lot. However, for One Piece there is an additional reason. This story takes some inspiration for characters from real-life pirates (and other individuals) which were primarily English, Spanish, French, and Portuguese. The characters are generally not Japanese.

Just focusing on Straw Hats:

Sanji, Robin, and Brook use french for their attacks. Robin also mixes in Spanish while Brook mixes in German. Franky is obviously an American and keeps speaking American English. Luffy, Usopp, Chopper, and Nami seem to be mostly English speaking. Basically, Oda sometimes borrows words from the languages he thinks are appropriate for that character whenever he wants to for emphasis. Zoro is thoroughly Japanese despite not being from Wano. Instead of languages, he names otherworldly imagery for emphasis.

Interestingly, this doesn't line up well with the nationalities in the SBS. Otherwise, Robin would speak Russian, Luffy would speak Portuguese, Usopp would speak an unspecified African language, and Nami would speak Swedish.

The Doflamingos use Spanish. The World Gov. uses English. This does not explain all uses of English and it does not explain them at all in universe.