How to change the default language of a wordpress site?

Question

WordPress

Hello.
I have a wordpress website with the loco translate plugin. And two languages ​​Ukrainian and Russian.
Now the main page is loaded in Russian, but it needs to be loaded in Ukrainian.
That is, main.com/index.php is the Russian version, but you need to change it to main.com/uk/main-ua
I did a redirect, but then the Ukrainian version is always loaded and it is impossible to change the language.
In the site settings, I set the default language of the site to Ukrainian, but this did not work, as it was loaded in Russian and loaded.
I set define('WP_HOME', ' main.com/uk/main-ua ') in wp_config, but it has no effect, main.com/index.php is loaded anyway .
In the database, in the wp_options table, I changed the settings, the links in the menu changed to the "ukr" version, but there is no text of the links, and the content of the main page remained "rus".
Anyone who has done this please advise. Thank you.