Is there any way to get a script to return to a certain line of code? (SOLVED)

Yes, the PlayerRemoving event would go there. When the playerremoving event fires, check if that player is the german leader. If it is, let the script know to start over. The script will know when to start over when it has finished waiting 600 seconds, OR if the german leader has left (you can just keep waiting in a while statement that checks both of these statements)

while true do
    if #game.Players:GetPlayers() >= 2 then --if the amount of players in this game is 2 or more
        --pick new players
        French = game.Teams.French:GetPlayers()
        Germans = game.Teams.Prussians:GetPlayers()

        GermanLeader = (Germans[ math.random(#Germans)])
        FrenchLeader = (French[ math.random(#French)])

        Tag = Instance.new("NumberValue", GermanLeader.Character)
        Tag.Name = "LeaderTag"
        TagScript = game.ServerScriptService.Holder.TagScript:Clone()
        TagScript.Parent = Tag
        TagScript.Disabled = false

        Tag1 = Instance.new("NumberValue", FrenchLeader.Character)
        Tag1.Name = "LeaderTag"
        TagScript1 = game.ServerScriptService.Holder.TagScript:Clone()
        TagScript1.Parent = Tag
        TagScript1.Disabled = false

        local left = false
        game.Players.PlayerRemoving:Connect(function(player) --when a player left
            if player ~= GermanLeader then --if the player that left is not the german leader, then
                return --you shall not pass
            end

            left = true --this player has left, let the loop know
        end)

        local timeThis = os.time()
        while (os.time() - timeThis) < 600 and left == false do --while this amount of seconds have passed is less than 600 and the player hasnt left yet,
            wait() --keep waiting
        end
	--we are done waiting, the loop will start over now
    end

    wait()
end

I have commented on what I edited.