One of Peter's rules was to prohibit speaking German at work. If your were caught you breaking the rule, you would get a gentle Teutonic reprimand. I heard some grumbling that Peter even enforced this rule when playing soccer at lunchtime.
Although German would have been comfortable for some people, it would have excluded others. Equally importantly, Peter wants even students who are not native English speakers to be perfectly comfortable in English — for example, so that they can network effectively at conference dinners.
Peter found he had to adjust his rule from "no German in the building" to "only English in the building" when he discovered people speaking Hindi in their offices.
During this year's sabbatical, at IMDEA, conversations were in a mix of Spanish and English, but mostly English.
At UBA, the technical work is mostly in Spanish, except when I am around. At lunchtime and during informal preliminaries, I speak Spanish, but I find it easier to be precise in English. The faculty I interact with are completely fluent in English, and they are glad that I am forcing the language to be English since that is good practice for the students. They enthusiastically adopted Peter's original "no German in the building" rule. (Only Sven Stork and I could have objected, and my German is too primitive for Sven to ever want to hear it!) But they weren't willing to upgrade to Peter's current "only English" rule.