Because the Spanish flag carries a coat of arms, because that coat of arms contains text, and because the coat of arms is not actually central on the flag, one can ask what the other side of the flag should look like
Heraldic arms should always face the same end of the flag on both sides of the flag, so if it faces the hoist on the front side, it should face the hoist on the reverse side of the flag too. On the front side of the flag is the State Arms with the heraldic right side (dexter) towards the hoist (in this case with the Arms of Castile nearest the upper hoist corner of the flag). On the reverse side the Arms should therefore be reversed, so what we see is a mirror image of the arms — with the arms of Castile nearest the upper hoist.
In practice this more often than not, is not the case. It would require a second design for the coat of arms. And if an embroidered coat of arms is used, then it will just be a duplicate, rather than a mirror, of the front of the flag.
If the coat of arms is woven into the flag, rather than an embroidered on sewn on, then it will be correctly positioned as a mirror on the reverse side. However the wording " "PLUS ULTRA" will be mirror lettering too, and therefore unreadable.
The Flag of Spain