Clear both text fields under Toggle Button → Button Text and save. Empty fields use the default wording, translated into each visitor's language. A field holding the default in any language the plugin comes in, such as "Modo oscuro", counts as empty too. Words you type are shown exactly as typed, in every language, unless you translate them: WPML String Translation and Polylang both list the two fields, and each visitor then sees their own language.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
