If there isn't a language key ('world' icon), most times long-pressing the space bar or long-pressing and swiping on the language options will prompt this. If this doesn't help, go to the Keyboard Settings ('cog' icon) and check your options in 'Languages and types'.
Also check your phone's Language settings (not the keyboard-specific) in case something there changed, but if this was the case you'd likely see other UI elements in French as well.
PS: I could have sworn I saw Samsung Keyboard somewhere, but that may not be the case. The "Languages and Types" option is specific for this, but most keyboards should have something similar, just maybe different wording.