Is a marriage considered valid if the guardian is not diligent in prayer and at times utters blasphemous statements, and if there was no verbal form of proposal and acceptance, but rather a signature on a written contract in a language the husband does not understand?

The aforementioned contract is valid because the guardian is a Muslim. Even if he apostatized and then prayed, his Islam would be affirmed. His being a fasiq (transgressor) by sometimes abandoning prayer does not invalidate his guardianship. Furthermore, a marriage contract is concluded by the words and actions customarily recognized by people, and there is no requirement for a specific order between the offer and acceptance.