In a soup pot, combine the stock with the lemongrass, ginger, jalapeño and garlic and bring to a boil. Add the onion, carrots and celery and season with salt and pepper. Cover and cook over moderate heat until the vegetables are nearly tender, about 5 minutes. Add the potatoes, cover and cook until all the vegetables are tender, about 15 minutes longer. Add the chicken and simmer just until warmed through. Remove the lemongrass and ginger and stir in the mint. Serve the soup in deep bowls.