Unable to change language in OpenCart 4.0.1.1 - 4.0.2.1

We develop modules for OpenCart 4.0 and noticed that the OpenCart 4.0.1.1, 4.0.2.0, 4.0.2.1 releases have many flaws, one of which is the issue with language switching. This issue does not appear if the site has only one language, but if there are several languages you can notice that changing the admin language is impossible, or that the standard module phrases are not translated on the site when the language is changed - they are always displayed in English. Of course, this is a serious OpenCart bug, which, however, its author is in no hurry to fix despite numerous complaints from users and contributors of the system. We hope that the issue will be finally resolved in upcoming OpenCart releases, and for now we offer a solution by editing system files.

The admin language does not change in OpenCart 4.0.1.1 - 4.0.2.1

In the admin panel header there is a language switcher that displays all available locales. But when you try to change the language in the admin panel, nothing happens. More precisely, the page reloads, but the language remains English. The problem is known and is solved by replacing a block of code in a single file.

Open the admin/controller/startup/language.php file and find the following block of code (if you have renamed the admin folder - look for the file in the folder with your new name):

// Language
if ($language_info['extension']) {
 self::$extension = $language_info['extension'];

 $this->language->addPath('extension/' . $language_info['extension'], DIR_EXTENSION . $language_info['extension'] . '/admin/language/');
}

Replace the above code block with the following code:

$this->language = new \Opencart\System\Library\Language($code);
// Language
if ($language_info['extension']) {
 self::$extension = $language_info['extension'];

 $this->language->addPath(DIR_EXTENSION . $language_info['extension'] . '/admin/language/');
} else {
 $this->language->addPath(DIR_LANGUAGE);
}

Module headings and phrases are not translated on the OpenCart 4.0.1.1 - 4.0.2.1 site

When changing the language on the site, some phrases may not be translated due to missing translations for specific phrases, but there is also a problem with translating the standard modules, for example such as Account or