Important Note: tax_query takes an array of tax query arguments arrays (it takes an array of arrays).
This construct allows you to query multiple taxonomies by using the relation parameter in the first (outer) array to describe the boolean relationship between the taxonomy arrays.
In 'tax_query' you should add array of ...
Since I have less reputation I can't comment, this is not answered with code but wanted to help, you may check this plugin: https://wordpress.org/plugins/redirect-404-error-page-to-homepage-or-custom-page/ or just take required code from the plugin ( I am not connected with that plugin anyway )
As SallyCJ mentioned, gettext is really the only option you have with this, because the Slug labels are hardcoded in the Core of WordPress.
Here is a snippet I wrote and tested that should work for you, and it will only run on the backend and only on the two pages where you ...