Update, but with some pending adjustments...
This was made possible by the advances of theFRANK and Rodrigovis.
Download: https://mega.nz/#!t8YhwQzS!uk-lcut6wVRQQiiBwB_omAdRnXXOBsBs0SFHXEklIRU
To install it, simply create a folder ui in GPBikes/gpbikes and drop the file "Portugues(PT).str" in it.
Then, launch the game, go to "Profiles" and change the language to "Portugues(PT)":