Why is there copying "I understand English..." in agreement or contract doent?

They put that there, so that they can ensure that you read and understood the contract. If you refuse to sign then they can either refuse to enter into said contract with you, or require that you have it translated before you sign. Either way it is for the protection of the person you are entering into the agreement with. They are probably tired of the, "I didn't understand English, so how do I know what the contract says" defense when they have to take people to court, because they break rules, or tear up the rental space.