וַיֹּאמְר֨וּ הָעָ֜ם שָׂרֵ֤י גִלְעָד֙ אִ֣ישׁ אֶל־רֵעֵ֔הוּ מִ֣י הָאִ֔ישׁ אֲשֶׁ֣ר יָחֵ֔ל לְהִלָּחֵ֖ם בִּבְנֵ֣י עַמּ֑וֹן יִֽהְיֶ֣ה לְרֹ֔אשׁ לְכֹ֖ל יֹשְׁבֵ֥י גִלְעָֽד׃
The troops—the officers of Gilead—said to one another, “Let whoever is the first to fight the Ammonites be chieftain over all the inhabitants of Gilead.”
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the Hebrew term containing אִישׁ.)
Prototypically, אִישׁ is used in sketching a situation schematically. Here, it performs its basic function of labeling the hypothetical yet situation-defining participant.
As for rendering into English, the NJPS ‘Let the man who is first’ is nowadays overly masculine in its connotation (given that in a military context, it goes without saying that women are not in view). The revised rendering is similar to REB, NLT, NIV.