So far, the methods related to text localization - `PHPMailer::setLanguage()`, `PHPMailer::getTranslations()` and `PHPMailer::lang()` - did not have dedicated tests.
The test file this commit introduces, tests all aspects of these methods, including the changes introduced in response to 2418 and 2419, as well as documents the current behaviour of the methods for specific, outlier situations.
So far, this method did not have dedicated tests.
The test file this commit introduces, tests nearly all aspects of the method as well as documents the current behaviour of the method.
There is one particular test case missing. This is annotated in the class docblock.
While this method is quite simple, testing it separately means that the tests for methods _using_ this method don't have to _also_ test the functioning of this method, which means they can be more focussed on their own logic.
