WordPress should always allow me to change the language, anytime. However, sometimes I get stuck in these kind of situations, where I only get to choose “English (United States)”.
(Settings > General Settings > Site Language)
What I expect:
What I get:
Why is that? How can I fix this?
Maybe WordPress doesn’t have permissions to save the new language files. I had the same problem, and I solved it by adding the following to wp-config.php
define('FS_METHOD', 'direct');
You might also want to check that your wp-content/-directory is writable for the web server user.