Hi Tilen,

each component in Joomla has single language file for front-end, for example,
language\en-GB\en-GB.com_sppagebuilder.ini

Use Joomla Language Override feature to change language phrase, do not touch .ini file.

;Optin Form;
 COM_SPPAGEBUILDER_ADDON_OPTIN_FORM_EMPTY_API="Please insert your API key for"
COM_SPPAGEBUILDER_ADDON_OPTIN_PLATFORM_EMAIL_PENDING="We need to confirm your email address. To complete the subscription process, please click the link in the email we just sent you."
COM_SPPAGEBUILDER_ADDON_OPTIN_PLATFORM_EMAIL_CONFIRMED="Your subscription to our list has been confirmed. Thank you for subscribing!"
COM_SPPAGEBUILDER_ADDON_OPTIN_PLATFORM_EMAIL_UPDATED="Your email was updated successfully."
COM_SPPAGEBUILDER_ADDON_OPTIN_PLATFORM_EMAIL_EXIST="You are already subscribed."
COM_SPPAGEBUILDER_ADDON_OPTIN_PLATFORM_EMAIL_ERROR="Some problem occurred, please try again."
COM_SPPAGEBUILDER_ADDON_OPTIN_PLATFORM_EMAIL_DUPLICATE_USER="This user is already exist. Please try with new email."

For "email" placeholder phrase use & override that one: COM_SPPAGEBUILDER_ADDON_AJAX_CONTACT_EMAIL

Newsletter addon do not have <label> field only placeholder that is displayed inside <input> field.