With the introduction of Accountwide Achivements you don't really care, who was the first character you got an achievement with?

Then ExoY's Achievement Anonymizer is what you are looking for!

It will hide all instances within the achievement tab which states: "Earned by...."
It will work for any languages but the menu is only available in english at the moment.

If you looking for a little more considerate approach, check out Honestly-Earned Achievements

As a little gimmick you can also put there a little text of your own choosing with customizable color.

Dependencies: LibAddonMenu-2.0, LibExoYsUtilities