The inspiration [for the Je suis Charlie slogan], obviously, is the famous “I’m Spartacus” scene in Stanley Kubrick’s 1960 movie Spartacus. But Je suis Charlie is unlike I’m Spartacus in one respect: it’s ambiguous. The verb être (“be”) and the verb suivre (“follow”) share the first-person singular present tense form suis, so Je suis Charlie can be read either as the mostly intended defiant moi aussi solidarity claim (I am Charlie too, and if you attack the magazine Charlie Hebdo you attack me), or as simply “I follow Charlie.” The two meanings could be intended simultaneously, of course.
The ambiguity is only there in the first-person singular. If Language Log wanted to say “We are Charlie”, taking the side of Charlie Hebdo rather than the side of the cold-blooded murderers who slaughtered the staff in the magazine’s weekly editorial conference, the French would be Nous sommes Charlie, and it would mean only “We are Charlie”, not “We follow Charlie.”

My brother Sasha adds,

Just goes to show how far French has fallen: “je suis, tu es, il est, nous sommes, vous êtes, ils sont” comes from “sum, es, est, sumus, estis, sunt”, and “je suis, tu suis, il suit, nous suivons, vous suivez, ils suivent” comes from popular (not Classical) Latin “sequo, sequis, sequit, sequimus, sequitis, sequunt”. But now “sum” and “sequo” are basically the same….
Note that the classical Latin form is “sequor, sequeris, sequitur, sequimur, sequimini, sequuntur” (e.g. “non sequitur”), which is a deponent verb, meaning it looks just like the passive form but in fact it’s active (so, “amor, amaris, amatur” means “I am loved, you are loved, he is loved”). “Loquitur” is also an example of a deponent verb meaning “he speaks”, e.g. “res ipsa loquitur”. Just saying, because some commenters might claim that there’s no such form as “sequo, sequis, sequit” (from the infinitive “sequere”). It’s true that there’s no such form in classical Latin, but there was such a form in popular Latin, and a source I consulted tells me that it derives from the popular Latin form.

Interesting. And at least it’s better than Je sue Charlie.