We do have the ability to alter default language within your Constant Contact account. Although it seems like you are wanting the default text within emails to change as well. Unfortunately this is not the case. By changing the language in our system, it will make it so when you're typing in spanish, that the system isn't looking for English and marking those words as being mispelled. As certain parts about our campaigns are not editable and will remain in English, we didn't want to allow those areas that are editable to have language changes in case there was ever a case where two different languages were used by accident within the same campaign.
This is great feedback though, if you'd like to implement it Here, we would greatly appreciate it!