First of all, Brussels can no longer insist that this is an internal, domestic, Spanish issue, since Catalan president Puidgemont is in... Brussels. So are 4 members of his government.
That moves decisions to be made about his situation from the Spanish legal system to its Belgian counterpart. And the two are not identical twins. Even if both countries are EU members. This may expose a very large European problem: the lack of equality among justice systems. Citizens of EU member countries are free to move and work across the Union, but they are subject to different laws and constitutions.
The way the Spanish government tries to go after Puidgemont is exactly the same as the way Turkish president Erdogan tries to get to his perceived archenemy, Fethullah Gülen, a longtime resident of Pennsylvania. But the US doesn't want to extradite Gülen, not even now Turkey arrests US embassy personnel. The Americans have had enough of Erdogan.