You see text in English when it should be in another language, or vice-versa.
Magento 2 uses a CSV file to configure translations, using a phrase as input and outputting the translated version of this phrase. These translations have to be in the file, otherwise the input text will be left untranslated.
Add your translation file to the directory https://github.com/Adyen/adyen-magento2/tree/develop/i18n. If the file is already there, add another entry at the end of it. Backup this entry as it can be overwritten or affect updates on the plugin.