waarom praat iedereen Engels hier?

Correct. Same as a creature with 0 toughness (which is not the same as a creature with 3 toughness and 3 damage on it). 0 toughness means a one-way ticket to the graveyard, indestructible or not.

Check the Indestructible page to see what it does and doesn't.

When the planeswalker has Loyalty 0 or less, it leaves the battlefield. Indestructible only prevents it from being destroyed by things as Vindicate.

What happands when you cast this on an planewalker. And the enemy attacks it and the planewalker reaches 0 counters. Does it remain on the field with 0 counters since it cannot be destroyed or does the planewalker die?