Translating a transform also rotates it (Unity 2017.3.1f1)

I'm building a little scene in 2D. My player game object is a sphere sprite and to it is attached the script that you can see below. The player can shoot projectiles which are visualized as a stretched sprite.

For now, I only want to place the projectile in correct orientation above, right, below, and left of the player game object. Because the projectile sprite has a horizontal orientation by default, I'm only modifying the rotation for the UpArrow and DownArrow keys.

Both when I only rotate the projectile transform (left picture below) and when I only translate it (right picture below), it works as expected. (On the left I selected the correctly oriented projectile sprite because it wouldn't be visible otherwise)
But when I rotate and then translate the projectile transform, the translation does not move it where it should but in the angle that I supplied for the rotation (intended position in red)

This script is attached to my player game object:

 public class Shoot : MonoBehaviour {
     
         public GameObject projectilePrefab;
         GameObject projectile = null;
     
         private void Update()
         {
             if (Input.GetKeyDown(KeyCode.RightArrow)){
                 projectile = Instantiate(projectilePrefab);
                 projectile.transform.Translate(Vector2.right);
             }
             if (Input.GetKeyDown(KeyCode.LeftArrow))
             {
                 projectile = Instantiate(projectilePrefab);
                 projectile.transform.Translate(Vector2.left);
             }
             if (Input.GetKeyDown(KeyCode.UpArrow))
             {
                 projectile = Instantiate(projectilePrefab);
                 projectile.transform.rotation = Quaternion.AngleAxis(90, Vector3.forward);
                 projectile.transform.Translate(Vector2.up);
             }
             if (Input.GetKeyDown(KeyCode.DownArrow))
             {
                 projectile = Instantiate(projectilePrefab);
                 projectile.transform.rotation = Quaternion.AngleAxis(90, Vector3.forward);
                 projectile.transform.Translate(Vector2.down);
             }
         }
 }