As of WordPress 5.0 there is a new filter, to get the language you need:
``` /** * Filters the locale for the current request prior to the default determination process. * * Using this filter allows to override the default logic, effectively short-circuiting the function. * * @since 5.0.0 * * @param string|null $locale The locale to return and short-circuit. Default null. */ $determined_locale = apply_filters( 'pre_determine_locale', null ); ```

As of WordPress 5.0 there is now a filter for this:
``` /** * Filters the locale for the current request prior to the default determination process. * * Using this filter allows to override the default logic, effectively short-circuiting the function. * * @since 5.0.0 * * @param string|null $locale The locale to return and short-circuit. Default null. */ $determined_locale = apply_filters( 'pre_determine_locale', null ); ```