Iam a teacher am new to App Inventor I have been trying to do an app that can allow me to upload or scan files and translate them to other language and give me a text or speech
We are not teachers who teach other teachers how to implement something without the teacher first having tried to teach themselves. 
So what have you tried so far?
To read a text file, which is not owned by your app, you need SAF, see also Some basics on Android storage system
See also
Taifun