If you want all text to be in French the best way would be to change it in the Paragraph Style definitions, but you can use Find/Change to override one language (look for it in the Find Format settings) and change to another (in the Change Format).
The Language setting is under Advanced Character Formats in the Paragraph Style panel.
Change it in the para style.
Or, if you need only certain words, make a charact-style attribute under the same sub-head.
Advanced Character Formats (that is "Formats de caractère avancés"). Very funny location. Thank you P eter and Sandee.
Very funny location
Language is a character level attribute so it can be assigned in a number of places—Character, Character Style, Paragraph Style, and Control panels all have language settings. If you have a long document with a mix it can be difficult to get it into a single language.
If you are using OSX this AppleScript will set everything (Paragraph Styles, Character Styles, and paragraphs) to French in an existing document with mixed languages:
set lang to "French"
tell application "Adobe InDesign CC 2014"
set pstyles to all paragraph styles of active document
repeat with i from 1 to number of items in pstyles
set applied language of item i of pstyles to lang
set cstyles to all character styles of active document
repeat with i from 1 to number of items in cstyles
set applied language of item i of cstyles to lang
set applied language of every paragraph of every story of active document to lang
set applied language of every paragraph of every cell of every table of every story of active document to lang