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Editing .inc files creates 500 error

I’m very new to this, so apologies if this all sounds basic. I’m using self-hosted Phplist on a shared server installed via cpanel.

I’d like to change my english.inc file to use custom language. I copied the entire english.inc file and created a new one titled english_nb_custom.inc (following the tutorial found here: Tutorial - Creating Custom Language File).

If I change to the new language file in the admin panel (Conifg > Subscribe Pages > Language file to use > english_nb_custom), I get a 500 error for my newsletter site, but NOT in the admin panel. If I change the language file in the config.php file (public_html > newsletter > config > config.php > $language_module > $language_module = ‘english_nb_custom.inc’;), I get a 500 error for the entire newsletter site, including the admin panel. The 500 error reverts after restoring the original english.inc file, or switching the Language file to use in the admin panel.

What am I doing wrong here?