If the text is not being translated but still showing in English, this indicates a problem with the translation files. 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.
In order to solve this issue, add your translation file to the directory at 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.