Sorry for the long post. Let a native speaker try to answer.
Jian 劍 refers to a double-edged sword, while Dao 刀 refers to a single-edged sword In Chinese. There may not be a single character that accurately serves as a general term for swords regardless of whether they’re double- or single-edged, so in many cases we directly combine the two characters into the compound 刀劍.
When translating English words that contain sword, such as swordsman, Chinese usually renders the sword part as jian, and then chooses the character for the man part according to context. For example, 劍士, 劍兵, 劍客, and 劍俠 are all commonly used to correspond to swordsman. In all of them, jian represents sword, while the character corresponding to man introduces subtle differences in meaning: roughly warrior, soldier, vagabond (I’m not entirely sure about this one in English), and folk hero, respectively.
Therefore, when a unit is called Jian Swordsman, a literal translation would become “劍劍士” (jian jian warrior), quite awkward. As a result, in Chinese localization it is actually named 吳越劍士, literally “Wuyue Swordsman”, creating distinction by adding a regional name at front. However, adding this regional association that does not exist in the original meaning makes it feel somewhat odd for other civs to use this unit.
In my suggestions for the Three Kingdoms civs (check the details), I would prefer to rename the Jian Swordsman as “Dare-to-die (死士/敢死軍).” Using the name of a unit that actually existed in the Wu Kingdom avoids the translation issues related to weapons altogether.