wp-config.php problem

I’m a complete noob, so please don’t be reluctant to explain at a VERY basic level. Feel free to write as if I’m really stupid too.

I’m trying to use a theme called binary-blue. This theme is reading in German even though the theme viewer test run showed that theme in English and other themes I’ve tried showed up in English just fine when I put them on my site. By reading the “readme” text I believe I have found the explanation (see especially the last sentence of the quotation):

“IMPORTANT!!!! Make sure you have set up your wordpress
installation correctly, especially the WPLANG! This variable MUST have a valid value like EN_US, ZH_CN, ES_ES and so on, otherwise it may happen that your wordpress installation will produce errors like the well-known
Fatal error: Cannot instantiate non-existent class:
cachedfilereader in /some/directory/structure/here/wp-includes/wp-l10n.php on line 72
It is ABSOLUTELY MANDATORY for the Binary Blue theme to have a valid locale definition in your wp-config.php, as the theme is fully localized and therefore depends on a correct setup! Because this is
the most commonly known issue with localized themes and plugins, I have now – finally – implemented a check routine that will look whether the WPLANG is set with a non-empty value, and if this check fails, the complete theme will fall back to its native German language.”

So apparently the WPLANG is set up wrong in my wp-config.php

How do I make the fix please.