this is getting fixed right now

It seems to be missing from a few files, here's where it's present in the interface directory:

./gateway_common/i18n/interface/sv.json: "donate_interface-email-explain": "Vi kommer att mejla dig ett kvitto för att konfirmera att vi tagit emot ditt bidrag.",
./gateway_common/i18n/interface/nb.json: "donate_interface-email-explain": "Vi vil sende deg en kvittering for å bekrefte donasjonen.",
./gateway_common/i18n/interface/zh-hant.json: "donate_interface-email-explain": "我們會電郵給您一份收據來確認您的捐款。",
./gateway_common/i18n/interface/zh-hans.json: "donate_interface-email-explain": "我们会发送您一份电子收据以确认您的捐赠。",
./gateway_common/i18n/interface/he.json: "donate_interface-email-explain": "אנחנו נשלח לך קבלה שתאשר את התרומה שלך.",
./gateway_common/i18n/interface/ko.json: "donate_interface-email-explain": "기부 확인을 위한 영수증을 이메일로 전달하겠습니다.",
./gateway_common/i18n/interface/id.json: "donate_interface-email-explain": "Kami akan menyurelkan Anda tanda terima untuk mengkonfirmasi donasi Anda.",
./gateway_common/i18n/interface/ar.json: "donate_interface-email-explain": "سنرسل إليك إيصالا عبر البريد الإلكتروني لتأكيد تبرعك.",
./gateway_common/i18n/interface/qqq.json: "donate_interface-email-explain": "A message explaining why we need an email address, even when the donor does not want to be on our mailing list",
./gateway_common/i18n/interface/de.json: "donate_interface-email-explain": "Wir senden Ihnen eine Quittung per E-Mail zur Bestätigung Ihrer Spende.",
./gateway_common/i18n/interface/fr.json: "donate_interface-email-explain": "Nous vous enverrons un courriel de récépissé pour confirmer votre don.",
./gateway_common/i18n/interface/pt.json: "donate_interface-email-explain": "Enviaremos um recibo por correio eletrónico para confirmar o seu donativo.",
./gateway_common/i18n/interface/uk.json: "donate_interface-email-explain": "Ми надішлемо вам квитанцію електронною поштою, щоб підтвердити вашу пожертву.",
./gateway_common/i18n/interface/roa-tara.json: "donate_interface-email-explain": "Nuje te manname 'na mail pe ricezione d'a conferme d'a donaziona toje",
./gateway_common/i18n/interface/pt-br.json: "donate_interface-email-explain": "Enviaremos um recibo por e-mail para confirmar sua doação.",
./gateway_common/i18n/interface/nl.json: "donate_interface-email-explain": "We zullen u een ontvangstbewijs mailen om uw donatie te bevestigen.",
./gateway_common/i18n/interface/en.json: "donate_interface-email-explain": "We will email you a receipt to confirm your donation.",
./gateway_common/i18n/interface/eo.json: "donate_interface-email-explain": "Ni kvitancos vian donacon retpoŝte.",
./gateway_common/i18n/interface/mk.json: "donate_interface-email-explain": "Ќе ви испратиме потврда за даруваните средства по е-пошта.",
./gateway_common/i18n/interface/bg.json: "donate_interface-email-explain": "Ще Ви изпратим потвърждение на дарените средства по електронна поща.",
./gateway_common/i18n/interface/tr.json: "donate_interface-email-explain": "Bağışınızı onaylamak için size bir makbuz göndereceğiz.",

Change 601754 had a related patch set uploaded (by Ejegg; owner: Ejegg):
[mediawiki/extensions/DonationInterface@master] Add Spanish translation for email-explain

https://gerrit.wikimedia.org/r/601754

Change 601754 merged by jenkins-bot:
[mediawiki/extensions/DonationInterface@master] Add Spanish translation for email-explain

https://gerrit.wikimedia.org/r/601754

This is deployed and shows up fine. It does stretch into a second line though. I could take out the 'por correo electrónico' to make it more concise since that's kind of implied by the notice being under the email field.

adding @MSuijkerbuijk_WMF to confirm if this is okay.

Mariana, Elliot suggests removing part of the translation you provided so that it can fit onto a single line, so it would be:
Te enviaremos un recibo para confirmar tu donación.

See his note above. Is this okay?

OT re image in the description: The phrase Monto De La Donación is orthographically incorrect; should be Monto de la donación. Also, Monto sounds weird in Europe, unless that was the es-419 interface (Importe de la donación sounds more natural).

Same with Datos De Facturación --> Datos de facturación.

OK, I just deployed the shorter version of the email field explanation.

@Ejegg @MNoorWMF

I don't think that works because I understood that we need to explain/mention that even if they unsubscribe they'll get an email, right?

I'm sharing a few options below --including the word 'correo' for email--:

Elliot's suggestion:
Te enviaremos un recibo para confirmar tu donación.

Recibirás un correo con un recibo de tu donación.
Te enviaremos un correo con un recibo de tu donación.

Longer:
Recibirás un correo con un recibo para confirmar tu donación.
Te enviaremos un correo con un recibo para confirmar tu donación.

@MNoorWMF I agree with @MarcoAurelio --Monto does not work in Spain, Spanish. It should be Importe

I got a comment on my patch directly updating the message, asking to do translations through translate-wiki when possible. I guess the bidirectional sync isn't perfect.

Here's a link to show the message keys for each field in the forms. The magic is in setting uselang and language parameters to 'qqx':
https://payments.wikimedia.org/index.php?title=Special:IngenicoGateway&appeal=JimmyQuote&ffname=cc-vmad&payment_method=cc&recurring=&uselang=qqx&language=qqx&currency=USD&amount=1&country=US&optin=0

Here's the link to see all DonationInterface messages on translatewiki:
https://translatewiki.net/w/i.php?title=Special:Translate&group=ext-di-gc&language=es&filter=&optional=1&action=translate

Once any desired changes are submitted there, please give fr-tech a heads up and we'll watch for the bot to commit the changes to DonationInterface so we can deploy them.

Hi @Ejegg - I submitted some updates to translatewiki.net yesterday and those where committed as https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/DonationInterface/+/601965/ to the master branch of the DonationInterface extension. Thanks.

Thanks @MarcoAurelio ! I've just deployed those updates.

@Ejegg Thanks. Is there any HTML that makes the titles to use CamelCase?

See https://payments.wikimedia.org/index.php?title=Special:IngenicoGateway&language=es&country=ES

While the words for es were indeed updated, it's still using CamelCase which is incorrect for Spanish.

Thank you Elliot!