I18n form fix

Take note that only 'email' and 'body' are validated in the contact
form, so we must use a name attribute set to 'email' and 'body'
respectively for those. Every other field can use any other name
attribute. Shopify has no particular concept of a 'phone' field or
'name' field. So use localization there! These fields end up in the email sent to the merchant, who will appreciate having those in his/her own language. 

Also I added placeholder translations, since the contact form fields were re-using the label as
placeholder, which I think may have been “lazy”-ish of us.

@cshold @jonasll