OpenCart has a feature to detect the language of your browser and force that language by default.
Your browser knows your preferred language. Not only that, it is sending this information with every page request you make to a server. So the server also knows your browser language.
OpenCart has a small piece of code in
catalog/controller/startup/startup.php on line 56 where it tries to pull the list of languages from the header request and compare it to the available languages in the store.
If it finds a match, it will force this language onto the user, ignoring the default language set in OpenCart admin panel.
This is why, when a user visits your store for the first time, they may see your store not in the default language, but in their browser language.
This can be very confusing since most of OpenCart users have never heard of this feature and it is not explained in the documentation.
To turn this feature off, simply prevent the code from ever checking the browser language in the first place.
Just by replacing the if statement with false, we will skip the language to detect code and continue on, setting the default language as the current language.
If you don't like coding, use our OCmod file that we created for you. You can donload the code from GitHub and install it via Extension Installer.