By saying that it is not useful to translate certain parts of Launchpad you deny people who are not so proficient in English access to this project management website. An all English website puts people off, even if they can speak English.
When your English isn't very good it costs a lot of energy to read and write it. When you'd only have to write it, but can read the interface in your native language, things are much easier.
Also see <http://sensehofstede.nl/realise-native-english-speakers-are-privileged> for more arguments why the use of English can be a hindrance to contributions.
On top of that the LoCo Council seems to be encouraging Ubuntu LoCo teams to use Launchpad as much as possible. That does mean that the English-speaking LoCo teams have a huge advantage since they have this great tool at their disposition. For non-English LoCo teams, however, it is not that easy to use Launchpad. Many people in the LoCo teams are not that proficient in English at all.
One of the most important reasons why we're using Launchpad only very little in Ubuntu NL is because of the fact that it is fully English. Not everyone can use that language or wants to use it.
By keeping the code and bugs section English you also make it harder for a certain group of people to contribute (code) to the projects on Launchpad. Say someone can read some English but has trouble writing a lot of it. That means that the person can still perfectly contribute code. However, he'd have to use Launchpad in English, which is hard for him.
Native English speakers, please realise that not everyone finds using English as easy as you.