No, it isn't "griefing".
First of all Blizzard does not use or recognize the term "griefing". You won't find it anywhere in the in-game policy. Yes, Blizzard uses the term "griefing". I was wrong.
Secondly leaving a group, for any reason whatsoever, is fully within the right of a player, as well as it is the right to kick a player for any reason whatsoever.
Let me quote you Blizzard's in-game policy:
"Player Left Mythic Dungeon Before completing
Updated: 2 months ago
Article ID: 117469
Relevant Products:
Common Problems
Group wiped while doing a mythic plus and the player we invited to our group left
I just started a mythic plus run and a member left
Customer Support does not upgrade, restore, or create keystones. Running the dungeon again with the downgraded keystone will award a new upgraded keystone, as long as finished within the time limit."
https://eu.battle.net/support/en/article/117469
Combine that with Blizzard's policy on removing/kicking players:
"Groups are empowered to manage their own members through the vote-kick system. If the majority of the group feels a player should be removed, they can initiate a vote-kick to remove them. The system is entirely under the control of our players. Players may choose to remove any other player from the group for any reason, provided the vote to kick passes.
Note: Groups formed outside of the queue system (Guild Runs, Mythic +, etc.) will not utilize the vote-kick system. Instead those groups will have one party leader that controls who gets invited or removed."
https://us.battle.net/support/en/article/52363
What can we conclude from the articles:
As long as you don't use language banned by Blizzard, then you can do what the hell you want in a dungeon group: Kick, remove or leave.
So leaving a group or kicking someone is not considered "griefing" by Blizzard.
To the OP:
If you really think that players from certain servers/countries are worse than others then why the hell do you invite them?
I would never invite anyone from the Italian or Spanish server(s). Complaining won't get you anywhere. Stop inviting people that lessen you in-game experience.