blank page after switching language

Working on v2.1.0 on local (php 8.2) I installed bootstrap template, and cloned it, they would work fine with the default language set to English, when I changed default to Italian the page went blank, it stayed blank even when changing back to English.
It works fine with the responsive classic, but not bootstrap or its clone; no changes to either of them.

Peculiar fact is that I did the same on a live server, and it does not go blank, and works fine with both bootstrap, and its clone.

The log file reports:
PHP Fatal error: Uncaught Error: Undefined constant "TEXT_HEADER_ARIA_LABEL_NAVBAR" in C:\path-to\zencart\includes\templates\bootstrap\common\tpl_header.php:25

Not a big problem, as I can work on the live server, just curious to understand why in local behaves like this.

On your local site, check to see if any files named header.php (not lang.header.php) exist in any of these directories, where bootstrap-clone is the name of your cloned bootstrap template:

/includes/languages/english
/includes/languages/italian
/includes/languages/english/bootstrap-clone
/includes/languages/italian/bootstrap-clone

Thanks.
No header.php or lang.header.php in the directories you suggested, however it put me on the right track, perhaps I was tired when adding the Italian package, and missed to copy it into the extra_definitions directory, did so, and all is fine.