Re checks - should not be a problem, you can use Payment Instructions field to explain to people that if they want to pay by check, they should register but not go through the online payment. They will show up as "pending" and you will change status into "confirmed" once you get the check. Same for membership dues.
Also, you can log out from your administrator access and register - "pretending" to be that person. We will be adding functionality for admin to register event attendees soon.
We will be introducing some ways to simplify this process too.
Re membership history - you can put it into notes, would it be sufficient?
Renewals - each member can renew in his own profile and we will be adding more robust renewal workflow in the next 2--3 months.
Spanish support - not something we can consider at the moment, sorry.
Dmitry Buterin, Chief Apricot