Steps to reproduce the issue

Install Joomla 5.1 or upgrade to 5.1
Create a bilingual site
Create an instance of mod_languages in a position that exists also on an error page
In global configuration, set error reporting to maximum
Navigate to a url that will produce an error

Expected result

The module works correctly

Actual result

This warning message is displayed:

Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /home/gdwtest2/domains/gdwtest2.geantduweb.ca/public_html/libraries/src/Extension/ExtensionManagerTrait.php on line 45

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /home/gdwtest2/domains/gdwtest2.geantduweb.ca/public_html/modules/mod_languages/src/Helper/LanguagesHelper.php on line 79

Additional comments

I've tested this in both a freshly created site and a site that was upgraded to 5.1, the error occurs either way.