Defends - has contradictory meanings
Defends means defend and forbid, so in this sentence
Je vous defends de dire, which means I forbid you to say
How do I know that it doesn't mean, I defend you to say
Partly, it's the context. How the prepositions are used makes a difference as well. Word reference is helpful in seeing how words are used in various contexts. http://www.wordreference.com/fren/d%C3%A9fendre
In this case, "I defend you to say" seems to be missing some words in English. However, you could say, "I defend your right to speak."
Well in French they say "qu'est-ce que c'est" to mean "what is that" when the literal translation is "what is it that it is". Also they say "quoi que" to mean "whatever" when the literal translation is "what that" and "quand meme" to mean "nevertheless" when the literal translation is "when same". and to use a one word example I read in book "j'ignorais que" to mean "I did not know that" when the literal translation would be "I am ignorant of that." I could go on an on, but you can see I can't really look to english to see if it makes sense to determine what word it should be, because the French just express themselves differently. So I could think, this is not how I would word it in english but maybe it is in French. I guess this is a case where you have to be an expert in the language to know the difference.
By the way all these examples above are mostly, with the exception of the glairing example of qu'est-ce que c'est, from the last few pages of the book I'm reading.
In the link I provided in the first message, I found this sentence: "Sophie défend toujours son frère, même quand il fait les pires bêtises." = Sophie always defends her brother... Notice that there is no "de" after défend in that example.
Prepositions can often change the meanings of other words. So, I would assume that "Je vous defends de..." does mean "I forbid you.... "
According to this website (and the way I've always heard it used), "défendre (à quelqu’un) de" means "to forbid (someone) to do something," https://www.lawlessfrench.com/grammar/verbs-with-de/
Although I'm not an expert, I've spent a lot of time learning the difference between experessions like "une tasse à thé" = a teacup but "une tasse de thé" = a cup of tea. I also absolutely agree with you that literal translations are not always best.