The word ‘Jedi’ can be singular or plural, and these translated titles confirm ‘The Last Jedi’ it is one.
The theory originates from the fact that “Jedi” can be singular or plural, meaning the title could either refer to an individual character like Rey or Luke Skywalker, or to multiple Jedi.
The Brazilian translation seemingly suggested a singular, male Jedi when the title was first announced. However, several new translated titles were just revealed on official Star Wars social media accounts, including plural translations like the Italian Gli Ultimi Jedi.
It’s most likely both.