[when ye shall come unto me]: before you die.
[ye shall write them and shall seal them up, that no one can interpret them]: The sealing is one of sealed understanding, not a physical sealing. The things that were written were effectively sealed until Mosiah interpreted them.
[ye shall write them in a language that they cannot be read]: This is the same statement that Moroni has about his own writings (see Mormon 9:34). These are sealed records that cannot be read by typical human means.