When you edit the name of a chest boat in the lang file, it affects its name on the inventory UI name and on its tooltip. There is only one identifier that is used for both parts.

In this example, I changed the name of the cherry chest boat adding a bold style and a black color, like this:

  "item.minecraft.cherry_chest_boat": "§0§lCherry Boat with Chest", 

And this is the result:

This doesn't happen with a normal chest, because there are identifiers for both "block.minecraft.chest" and "container.chest", which allows for them to be changed freely and separately:

Thank you!