If two people are trying to create the same article in ContentTranslation, the second user will see a notification about this.

However, if a user is trying to create an article in one of the usual from-scratch editors, he won't know that somebody is already translating the same article. For that matter, that user won't know if somebody is already writing an article in a from-scratch editor either.

So some kind of a notification could be shown, though it probably shouldn't be blocking.