I think I read this over a decade ago in high school (1990s approximately), probably as part of an anthology, although I have no idea which. A few salient details have stuck in my mind. The main character is a male human who has had surgical modifications to look like the alien race, I believe infiltrating on the behalf of his government. There's some sort of ritual battle that he's involved in repeatedly involving a sort of sword that is flexible and can be re-configured into different shapes. I want to say that it's a mechanical rather than an electronic process to change the sword's shape, and I specifically recall him turning it into a hook shape. Near the end of the story, he's defeated by an opponent who turns out to be a human woman (probably working for an opposing side) and he reflects that she probably won because she had made more of a sacrifice with her surgery. I remember a description of milk ducts hanging loose due to her surgery.
I think you read "Single Combat" by Joseph Green.
The story is about a man sent to a planet with a primitive civilization in order to help them build their civilization before another race from another planet comes to take them over.
The central character is a large man named Kala Brabant from somewhere in Africa who was chosen for two reasons. One, his size and coloring make it easy for him to be surgically modified to pass for a native of the planet he is being sent to. The other thing is that he has a very high rating for psionics.
Each year at some particular festival, the ruler of one of the provinces must take on all challengers in mortal comabt. If the current ruler loses, then the challenger becomes the new ruler. Brabant becomes the ruler that way, then uses his position to start joining the other tribes together into one nation.
The story "Single Combat" tells all of this background during the events of the current day of challenge. One of the challengers turns out to be from that other race that wants to conquer all planets. Brabant realizes this as the fight plays out, and also realizes that his challenger is female.
The battle is fought with swords, but most of the action is mental as Brabant and the challenger fight each other psionically. The line with "milk ducts hanging loose due to her surgery" has to do with an image that Brabant projects at the challenger to weaken her will by reminding her of what she has lost.
In the end, Brabant loses and is killed, but the back up earthman takes advantage of the last few minutes of the day to challenge the new ruler.
It lacks the flexible sword, but has everything else.