Many programming teams I know operate in two (natural) languages most of the time: English most things online, German for inner-Team discussion, or in fact any non-technical communication. This especially holds true for communication with people that are not part of the team.
Luckily, I found another bit of argument for my point: Gherkin, a language (heh) to write tests as behaviours, advocates to write the behaviours in the user’s language:
The language you choose for Gherkin should be the same language your users and domain experts use when they talk about the domain. Translating between two languages should be avoided.
They are obviously serious about this: Gherkin scripts can be written in 70 natural languages.