And if the monk's mechanics match 100% what I want? If my view of the character includes the ki powers, but flavored as espionage-based mystical/martial training, and includes speaking all languages due to some magical ritual, etc., you'd still rather I not use the monk chassis because the flavor doesn't match?
Yep. Though at that point, I'd be wondering why it's so important that it NOT be consistent with the monk flavor. Nothing stops monks from being an ascetic, disciplined sect of semi-mystical agents of espionage. Heck, shadow monks are almost made for the job.

If you're doing magical rituals and whatnot anyway, why draw a hard line between that and ki? At that point, what is the difference, functionally? And why would that matter? Why *can't* you just be a monk? A magical ritual that makes you learn all languages is A-OK, but developing that ability based on a disciplined mastery of an inner magical energy is off the table?
That makes no sense to me. It's simple to say "The character's abilities resemble those of a monk but don't come from the same tradition." It strikes me as far clunkier to try to recreate the monk with a rogue.
It seems much clunkier to me to say "my character is a spy who has abilities based on pure skill except for these things which are magical rituals that happen to me at a certain level, which, yeah, DM, by the way, is totally a thing in your world now, because that's the character I'm playing." Generally, I'd rather innovate than kludge like that.
So much easier to say "My character is a monk who left the monastery to become a high-powered magical spy for the Crown."
That's a character narrative with some roots! You've got at least two organizations you're linked to as a character (which means at least two evil plots that their enemies are in the process of performing), an interesting conflict between them (which gives me a third narrative with some deep character resonance: monastery vs. crown), and all sorts of motive to go do awesome things.
The fact that the monk has a specific place in your world doesn't have to mean there's no possible other place for someone with abilities that are the same mechanically but not in terms of fiction.
I like my mechanics and my fiction to reinforce each other and build off each other and work together to make a great play experience. Treating these things like there is a wall between them is a dissociation that I find harms the suspension of disbelief at the table.
That means that monks have a particular narrative context tied to their abilities. A monk's punches aren't just martial arts, they're
supernaturally-powered, they do more damage than mere fists, strike faster than human reflexes, are capable of being faster than a shot arrow, etc. The inner magic and awareness is what allows monks to do that. Without that training, you can't do that - you can be the world's greatest martial artist, and you won't be able to rival what the monks do. What they do is special. It has meaning. According to the rules of the world, you can't do what a monk can do just by being Charles Atlas - you need access to what they know and what they can teach you that you cannot get on your own.
Like, it matters in the story that Cyclops's laser vision doesn't work like Superman's laser vision, and if I were to put them into a game, I'd expect them to work differently because of that story.
Similarly, it matters that a monk's martial arts are powered by
ki, and other martial arts aren't. It matters in the story, and it should matter in the mechanics.
Which means that if you want to punch and kick without being a monk, we gotta find a way to represent what it is
for your character that makes it different, and how that is expressed in the fiction and in the world.
And if it's not really all that different, just be a friggin' monk, dude.