May I suggest that an addition be made to the https://www.duolingo.com/skill/fr/Plurals discussion that says how users can type guillemets? It mentions them, but with an English keyboard, there are no such keys. Instead, keyboard sequences can be used:
Windows users can type ALT-0171 for « and ALT-0187 for ».
I don't know how Mac users would do it.
On a Mac with a US English keyboard and the default “U.S.” input source, Option-\ is used for « and Option-| is used for ».
Single guillemets can also be typed on a Mac: Option-# is used for ‹ and Option-$ is used for ›.
If someone on Windows follows the EnableHexNumpad method described here, then single guillemets can be typed on Windows by Alt-+2039 for ‹ and Alt-+203A for ›. (This article states that a reboot rather than logging out and logging in is needed after the Windows Registry change in Windows 10 — scroll down to its “Using Unicode Characters” section.)
Well, there are English keyboards and there are English keyboards...
In Window, using the United States-International keyboard allows « and » to be entered with Alt Gr+[ and Alt Gr+]. (That is, press the left ALT key and the desired square bracket key at the same time.)
In addition, this keyboard allows you to type other characters like é, è, ê, ë, and ç.