Since your first choice for the formal study of another language was Spanish, your first encounter with the Latin expression res ipso(a) loquitir arrived a bit later, when you turned your attentions toward the broader interpretations of Law, with reference to civil and criminal proceedings as opposed to such laws as Ohm's. Boyle, Charles, and Newton, taking in the behavior of physical matter.
Nevertheless. "Don't mention it" is that lovely quirk of recognition of the gift of awareness. Not surprisingly, one of the first non-combative social things you learned in Spanish--you could already say such things as pinche, pendejo, and the less combative but nonetheless definitive largate, and you knew enough nuance to know that abuela is grandmother, but tu abuela! is an altogether different matter--was ni modo, the equivalent of "No big thing" or "Don't mention it."
Herein dwells one of the many exquisite anomalies lodged within the human species. Of course you want someone to mention it so that you can tell them they didn't have to, a mention being a short awareness of a kind, perhaps even grateful recognition.
After eons of evolution, most of our cultures continue to revolve about the basic unit of awareness, the obligation. "Much obliged, pardner," as the old Western meme goes, or "Obrigato," as they say in Portugal and Brazil.
"Now that you mention it--" is, by extension, an acknowledgment of an important, perhaps touchy matter to be introduced into the conversation, a sort of Marquis of Queensbury Rules recognition that we are still on polite, civil terms. And "Hardly worth a mention" signals pretty much the polar opposite, the thing that ought to speak for itself does in fact not speak for itself, at least not in sufficient detail to be considered satisfactory.
"I only mention this because--" must rank as one of the more self-exculpatory memes in any culture. "I am the messenger. Therefore, do not shoot me." Really?
A friend of yours, with some frequency, pounces on a certain category of newly published books to see if he is mentioned in the Index, which would seem to suggest his own work and opinions having formed a portion of the intellectual conversation in the present work. "Not a mention of me," he will say, the implications as plangent as a deep sigh on a dense, overcast night.
All of which, as James Joyce informed us in his mischievous opening/closing line of Finnegan's Wake," brings us by a commodious vicus of recirculation back to Howth Castle and Environs," which is to say returns us to the thing, the res, that speaks for itself.
Of the many things you consider yourself to have learned over the years, through direct observation or even more direct experience, one is relevant here: The thing does not speak for itself, and unless you are vigilant, neither do you.