Same thing and I see you have no answers after one year...
I have changed the language and there is no modification on the formular.
I change the files :
fr-Fr.plg_system_gkreservation.sys.ini
and
fr-Fr.plg_system_gkreservation.ini
With a translation from EN to FR.
PLG_GKRESERVATION_DATE_PLACEHOLDER="Date"
PLG_GKRESERVATION_TIME_PLACEHOLDER="Heure"
PLG_GKRESERVATION_SIZE_PLACEHOLDER="Nombre de personnes*"
PLG_GKRESERVATION_NAME_PLACEHOLDER="Nom"
PLG_GKRESERVATION_EMAIL_PLACEHOLDER="E-mail"
PLG_GKRESERVATION_PHONE_PLACEHOLDER="Téléphone"
PLG_GKRESERVATION_TITLE_PLACEHOLDER="Sujet"
PLG_GKRESERVATION_TEXT_PLACEHOLDER="Demande spéciale"
PLG_GKRESERVATION_SEND_BTN="Envoyer la réservation"
PLG_GKRESERVATION_RECAPTCHA_ERROR="Invalid reCAPTCHA code"
PLG_GKRESERVATION_MESSAGE_SENT_ERROR="An error occured during sending your message: "
PLG_GKRESERVATION_MESSAGE_SENT_INFO="Your message has been sent!"
PLG_GKRESERVATION_NO_RECIPENT_INFO="The owner of this website didn't specified the recipient of your message."
PLG_GKRESERVATION_STANDARD_SUBJECT="Message from reservation form on: "
PLG_GKRESERVATION_MESSAGE_EMPTY_ERROR="You didn't filled all necessary fields - all fields are required."
PLG_GKRESERVATION_NAME_LABEL="Nom: "
PLG_GKRESERVATION_EMAIL_LABEL="E-mail: "
PLG_GKRESERVATION_TEXT_LABEL="Texte: "
PLG_GKRESERVATION_DATE_LABEL="Date: "
PLG_GKRESERVATION_TIME_LABEL="Heure: "
PLG_GKRESERVATION_SIZE_LABEL="Nombre de personnes: "
PLG_GKRESERVATION_PHONE_LABEL="Téléphone: "
I have also change activated the language into the backoffice and also changed the file
/plugins/system/plg_gkreservation/plg_gkreservation.xml
like this
"<languages>
<language tag="fr-FR">language/fr-FR"/fr-FR.plg_system_gkreservation.ini</language>
<language tag="fr-FR">language/fr-FR/fr-FR.plg_system_gkreservation.sys.ini</language>
</languages>"
And it doesn't work.
Dont know what to do...