The default language on a self-hosted WordPress website (or blog) is United States English.
If you’re from the UK, for example, and prefer using words like “bin” instead of “trash” or “hi” instead of “howdy”, or you’re in another part of the world where the predominant language isn’t English, you might like to change the default language to something more appealing to you.
Back in the day, you’d need a plugin (What is a WordPress plugin?) and language packs to manage this. These days, switching language is as easy as choosing an alternative from a drop-down menu.
Bear in mind, changing the default language won’t affect your content. That remains in the language it was written. It only affects the language in the admin areas.
Here’s a sample of French WordPress.

How to change the default language on your WordPress website
Click over to Settings in the left-hand menu and choose General.
Scroll to the bottom of the page and make your choice from the Site Language drop-down menu.

There are a lot of languages to choose from, far too many for me to list here. Instead, I’ve taken a screenshot so you can see some of the available languages.

After you’ve made your choice, hit Save Changes. Your site should now be using the language you selected.
Hope you find this article useful.
there are situations where WP does not change the languages, even when it shows the desired language as available
you may select your language of choice and hit the save button and BUMP again it is gone back to English / US
this happens from time and I see no way to keep a constant setting…
Thanks for your comment, Connie.
This isn’t something I’ve come across before as I always work on English sites. I wonder if a plugin causes WordPress to switch back to English?