The letters should be signed.. but it would be better to find someone else (family member, friend) who can also write a letter of support for your marriage.
We signed and dated them at the end. And like sbwv09 said, also provided 10 letters of support from family and friends. 5 on his side, 5 on mine. That's not a requirement for spousal applications, keep in mind, but it certainly can help your case.
We signed ours and my mom wrote a very nice letter too. Hubby's family isn't close and both his parents are deceased. My only immediate family here is my mom.
You can do it, but include a letter from you stating the language translated and your name, and swearing that it is a true translation of the original.
any supporting documents that you send non english or french has to be noterized and translated by a certified translator in order to avoid any conflict and its a good idea to sign at the end of the document with the date of the signature.