My Player's Handbook, in French, as well as this other source, describes Combat Style (two-weapon fighting) as follows:
Si le rôdeur choisit le combat à deux armes, il peut se battre comme s'il possédait le don Combat à deux armes, même s'il n'en remplit pas les conditions.
Literal translation:
If the ranger chooses two-weapon fighting, he can fight as if he possessed the feat Two-Weapon Fighting, even if he doesn't fulfill its conditions.
According to this definition, logically, if the ranger can only fight as if he possessed the feat, then he doesn't actually have the feat, and thus cannot use it as a prerequisite for Two-Weapon Defense. To take the feat, the ranger would need to also get the feat Two-Weapon Fighting (which would bring no additional benefits apart from providing a prerequisite).
However, I researched a bit more and found the following (source):
If the ranger selects two-weapon combat, he is treated as having the Two-Weapon Fighting feat, even if he does not have the normal prerequisites for that feat.
According to this definition, though, the ranger is treated as having the feat in all aspects, and thus should logically be allowed to choose the feat Two-Weapon Defense, which would bestow its benefits normally (as long as the ranger is wearing a light armor or no armor at all).
The English Player's Handbook has the same description (p. 48).
Now I wonder if this is a translation difference, a small difference between book printings, or something else. Either way, what is the exact rule for this case?
Best Answer
The ranger’s class feature grants Two-Weapon Fighting, and meets that prereq
Two-Weapon Defense requires two things: the Two-Weapon Fighting feat, and Dexterity 15.
Taking the English translation as authoritative (since the original authors were working in English), the ranger class allows you to get the Two-Weapon Fighting feat as a bonus feat, that is, you actually get the feat. It has never been otherwise in any English edition of the book; the wording in French is purely an issue of translation.
Furthermore, the ranger allows you to get the feat even if you do not meet its requirement: Dexterity 15, but restricts the armor you can use at the same time.
However, the ranger class does not waive the Dexterity 15 requirement of Two-Weapon Defense. Thus, if you are a ranger with Dexterity less than 15, you do not qualify for Two-Weapon Defense: you have Two-Weapon Fighting, but not Dexterity 15, and Two-Weapon Defense requires you to have both.
Moreover, if you do have Dexterity 15, and you take Two-Weapon Defense, it effectively inherits the armor restriction of the ranger’s Two-Weapon Fighting. Because wearing heavier armor means you lose Two-Weapon Fighting, you would lose the requirement for Two-Weapon Defense and thus be unable to use it. Probably won’t come up, unless someone forces you to wear armor somehow, but worth knowing.
As a complete aside, it may be worth pointing out that Two-Weapon Defense is an awful feat, and not worth taking. Shield bonuses to AC don’t apply to touch AC, only to regular (armored) AC, and that is not very valuable at all. Even if it was, +1 is a tiny bonus. Not in any way worth a feat.