To search for French-language sources in OneSearch, change the language drop down in the top search box to "French."
To retrieve sources in both French and English, scroll down to the language section in the left search bar. Hover to the left of each language to bring up the check box, and click there to include a language. Click save to update your results list.
To search for a term or idea in English and French at the same time, put the English phrase on one line in the top search box and the French in the line below it. Change the drop down on the far left of the second line to "OR" to find either of the two sets of keywords in a single results list.