Solution for server error 500

It lasted many hours to find all the pieces, but this solved my problem.

Before activation of Polylang: I tried to change the existing TwentyFifteen WP theme (with my own language: nl) to another theme (Odio): no problem. Everything works just fine!

I returned to the TwentyFifteen WP theme. After activating Polylang and translating every item in 2 languages: The site works fine.
I tried again to change the existing TwentyFifteen WP theme (with two languages: en + nl) to Odio (by activating it in the Themes window) and I got immediately a ‘Server error 500’…!

There are no plugins activated besides Polylang. Everything is up to date (WP 4.4.2, PHP 5.5.31).

I tried to fix it:
(1) Follow these steps: How to Deactivate All Plugins When Not Able to Access WP-Admin and
(2) switch WordPress debug mode to ‘on’ (procedure)

I entered my site again: (mydomain)/wp-admin. And I see on the Plugin screen:

The following required plugins are currently inactive: Odio Essentials and Visual Content Composer.

So, I activate the two required plugins. (I don’t activate other plugins yet).

I activate Polylang, and this is the error string as a result of the debugging:

Fatal error: Call to a member function get_default_language() on a non-object in (mydomain)/public_html/wp-content/themes/odio/functions.php on line 11

The cause for this error is a WPML ready theme (like Odio) that uses an internal WPML function without testing the existence of it.

I asked the theme developer to make his theme also ready for Polylang, but there is something you can do to make a WPML ready theme Polylang compatible. You find the solution here.

It worked for me. All the luck!

https://wordpress.org/plugins/polylang/