There's actually no such language as "Celtic". Celtic refers to a group of dozens of languages, six of which are spoken today:
A Celtic player by the name of Danny Invincible is not invincible
invincible
Invincible is an adjective.
The Latin translation of 'invincible' is 'invictus'.
Unconquerable or Invincible.
The invincible hero killed the gang of villians.
unconquerable
One word with the root 'vinc' is "convince," meaning to persuade or bring someone to a belief or opinion.
Muteki
The root word for "invincible" is "vincere," which is Latin for "to conquer" or "to win."
Celtaidd = Celtic
The villain stole many powerful weapons and is now invincible.