In the beginning of the book Ford Prefect uses language to persuade the builders to not knock down Arthur's house. He walks through all of the builders to see Arthur laying on the ground in front of a bulldozer. He says to Arthur "I need to take you to the pub to get a drink" Arthur says back "Cant you see I am laying in front of a bulldozer trying to stop them from knocking my house down". Ford somehow uses language to persuade the builders not to knock the house down while they go to the pub. He says "If your just going to stand around here and do nothing you don't actually need us". So Ford and Arthur go down to the pub because ford used language as a weapon to stop the builders from knocking down Arthur's house for a few minutes.
A part of the book where language fails Ford and Arthur is when the Vogon leader starts to read Ford his poetry. Vogon poetry is the 3rd worst poetry in the entire universe. So when the Vogon leader read Ford and Arthur his poetry Ford could not think of anything to say, he was speechless. After the Vogon leader finished he asked them to rate his poetry. Arthur said it was great and started giving a whole lot of reasons why it was so good. Ford was confused but then he got it so he joined in. They both complemented his poetry but it didn't work. The Vogon leader had one of his Vogon officers throw them off the ship into deep deep space.