Summary of Changes

Right now within Joomla\CMS\Language\Language when loading a default language's strings, those strings are mixed in with the strings of the active language. While the behavior is technically correct, this results in two message catalogues being mixed into one with data being overwritten during the various calls. Instead of doing this, a more preferential behavior is to treat each language's strings as a separate message catalogue, and that is what this PR implements.

A new class property, $defaultStrings, is introduced which is similar to the existing $strings array except it will only hold the strings of the default language when those are loaded, resulting in the $strings array now only holding strings of the active language. Methods which do read/write operations to this array (the _ method handling converting language keys to translated strings, the load and loadLanguage methods when reading files into memory, and the hasKey method when checking for existence of a key) are updated to work with this new methodology of splitting strings into different properties.

Testing Instructions

Install a second language to your Joomla site and make that your active language. With this PR applied, the translation keys should continue to be displayed in your active language if available, fall back to English if not available (when debugging is not active, when active we will skip this check), and display the untranslated key if not present at all.

Documentation Changes Required

If accepted, anyone who has subclassed this class and is working with the $strings class property should update their code to be aware of the new $defaultStrings class property. If they don't, the worst thing that happens is they would no longer get the English language fallbacks as those won't be included in the existing property.