These are the names given to this part. Which one you choose may depend on which English speaking country you are translating for.

These Spanish tutorials show what the 'tapeta' is and how it is inserted into the trousers.
https://www.decosturasyotrascosas.com/2014/04/como-poner-una...
https://www.elbauldelacosturera.com/2013/01/aplicacion-de-ci...

This one calls it a fly shield
https://www.doinaalexei.com/beginnersewingtutorialblog/how-t...

This one - fly guard
https://www.seamwork.com/magazine/2017/09/how-to-sew-a-zippe...

This one - zip guard
https://sewjanetmoville.co.uk/learning/zip-guard-application...

This one - zip shield
https://sewandrew.com/2016/09/15/the-last-chinos-of-summer/

This one - placket
https://sewitwithlove.com/fly-front-zip-tutorial/

There is also the option of using zipper instead of zip eg zipper shield.

--------------------------------------------------
Note added at 2 hrs (2021-10-21 18:42:37 GMT)
--------------------------------------------------

"Fly shield: This is a clean finished piece (self finished or faced) that resembles the shape of the facing described below. The fly shield is placed on the garment's wrong side such as to provide stability and a protective shield between the zipper teeth and human skin. In simple words, when you unzip a fly front zipper, the fly shield is the back portion that opens along with the zipper providing a flap-like structure behind one of the zipper teeth rows. "
https://www.doinaalexei.com/beginnersewingtutorialblog/how-t...