The US does not have an official language even though almost everyone speaks English. There are many Spanish speakers in the US who immigrated from Mexico, especially in the states on the border. There are also some French speakers. In Louisiana there was a large French settlement at New Orleans, and we also get visitors from Quebec in Canada, which is a French speaking region.

I would suggest that French appears on the US and UK passports because of the historical importance of French as a diplomatic language. However, I can't be sure.

French was considered the " diplomatic language" , the Spanish empire had a huge presense in America at the time , so those could be two big reasons . We could also considered that both countries helped the USA to gain the independence in the war , so that could be a reason too. But probably the main reason is that the country doesn´t have a official language.

Further to Mr. Chambers' response. The INternational Civil Aviation Organization, (ICAO) which is the Organization that creates the letter codes for airports, publishes standards for passports. They call for English or French, or both. The reason is practicality. Gate agents at international airports cannot be familiar with all languages.