How to change the default button text "choose a file" on the widget?

You can text with the help of UPLOADCARE_LOCALE_TRANSLATIONS global variable. 

See the Pen How to change the default button text by uploadcare (@uploadcare) on CodePen.

 Tweak Files settings object to put a custom text in a single or multiple widget buttons.  Images settings object will overshadow Files if images-only attribute is set for a widget.