Presumably because the contracting parties want the contract to be interpreted and enforced by the courts of a country in which they both do business.

It's not unusual, I think, for big international companies that already have assets and legal advisers in the UK to specify English law and English courts for a contract, but if a small Greek and a small Italian company were to have a contract in English, it would, I imagine, just make a legal dispute much more dififcult and expensive to resolve (whether or not the contract specifies English law and English courts: different problems in each case).

Because a dispute needs to be heard in a court, at which point precedent and/or statute need to be considered, and if the contract is in a different language to either of those two then interpreting all of them together becomes impossible.

Because legally, the contract has to be in the jurisdiction's official language

That’s not true - you can have a contract in any language in Germany. But the first thing that a court adjucating any dispute would do is have a certified translator translate it into German - at your cost. And that translation will be out of your control. So it makes sense to have a German contract, so at least you know what the contract says going to court.

correct - all our actual investment documents are in English.

Plus, if during the certification of the translation it comes up that the document does not comply with the legal requirements, it may lead to a contract invalidation. It can be really expensive depending on the matter of the dispute.

You can choose both the law and forum in many types of commercial agreements. Typically international contracts will choose English law as the governing law.

If your company has a legal representation in the English speaking country, sure, go for it.

Obviously not true for many countries. In Sweden we often use English even for domestic contracts. I've seen employment agreements in English between a Swedish company and a Swedish employee.