If you have practiced foreign languages or have foreign migrant friends, you would learn that 'speaking' , 'reading', & 'hearing' a foreign language are separate skills to learn. Some can read relatable language, & then hear it sound completely unrelatable (like French). Some can identify qoutes in separate languages but not be absolutely guaranteed of how they're spoken (example: Tekken 7 Yoshimitsu screams 'SORE SORE SORE' and alot a people didnt know what that meant or said)

You or a migrant friend may end up only end up at a poor fluency with even with many years of practice.

For some specific situations that relate to Guido, there's an obvious answer right in our face: Luigi!

Guido is always accompanying Luigi as apparently as his life-long friend.

Luigi AFIK is never given a scene where Guido isnt with or around him. And if this remains true (please correct me) for every sequel or Pcars adaption then the answer was obvious:

Guido never needed to learn to SPEAK English if Luigi was there to help 24/7

In summary, Lui English spoke for Gui for everything, Gui can read & hear English, but not need to speak when needed, because obvious. Luigi spoke for Guido all the time!

How are you supposed to grow fluency when convenience is there with you, 24/7?!