One thing I realised is that the default language doesn´t apply to email templates. Despite I have spanish language as default in whmcs, if a new client logs in, the display language is spanish, but the emails are sent in english (it uses the default template from the table where language = '') unless client specifies in their profile spanish language instead of default.
A workaround for this, without having to copy/paste all templates over default ones is to run a query like this
Update
tblemailtemplates as t1
inner join (
select t2.name, t2.subject, t2.message
from tblemailtemplates as t2
where language = 'Spanish'
) as A on t1.name = A.name
set t1.subject = A.subject, t1.message = A.message
where language = '';