how can we switch the language after the install of this program???

This is not really the proper forum for this question, but look in Preferences ( <CTRL + p> by default):

You should have posted in the help sections of the forum. You didn’t say what version of audacity you have nor which is your operating system.

In Audacity 1.3.12, go to Edit menu → Preferences → Interface → Language
If you have it in a language you can’t understand, press Ctrl-P (or Cmd-P on Mac) then click on the fifth element on the list (left side of the window) and click on the middle drop-down menu and select the desired language.