Intento extension automatically detects the source language of a translated web page based on the language attribute of the page. If for some reason this attribute doesn't work for you, you may turn off autodetection and make Intento extension to remember selected source language.
In order to keep source language selection in extension:
1. Right-click on the Intento icon at the top of the browser window to the right of the address bar
2. Select Options from the context menu
3. Turn Remember selected source language switch on
From that point on Intento extension will remember your last choice of Page language
Still, have questions? We've recorded a tiny video for you: