[jukx]

You can just pass a function as a second argument to RegisterEvent:

local doStuff = function(self, event, ...)
    local servers = { 
        ["Aggra(Português)"] = "Portuguese", 
    }
    ...
end
 
localeicon:RegisterEvent("PLAYER_ENTERING_WORLD", doStuff)
localeicon:RegisterEvent("PLAYER_TARGET_CHANGED", doStuff)

---

edit for verbosity:

Right now you're passing function that's supposed to be executed as a callback to SetScript:

localeicon:RegisterEvent("PLAYER_ENTERING_WORLD")
localeicon:RegisterEvent("PLAYER_TARGET_CHANGED")
localeicon:SetScript("OnEvent", function(self, event, ...) 
 
    local servers = { 
        -- ...
    }
 
    if select(2, UnitName("target")) and select(2, UnitName("target")) ~= '' then
        -- ...
    end
end)

Since oUF rewrote the RegisterEvent function, this might not work out as planned. Instead, you can just define that function locally and pass it as a second argument to RegisterEvent() like I showed above:

local setRealmIcon = function(self, event, ...)
    local servers = { 
        -- ...
    }
 
    if select(2, UnitName("target")) and select(2, UnitName("target")) ~= '' then
        -- ...
    end
end
 
localeicon:RegisterEvent("PLAYER_ENTERING_WORLD", setRealmIcon)
localeicon:RegisterEvent("PLAYER_TARGET_CHANGED", setRealmIcon)