Sorry - wasn't clear.
The company's policy is that English is the working language because it is an international company with teams all over the world - it just happens to have HQ in France. It is not a French company.
So in practice, people will speak their own language in their own offices, (likewise if they are speaking to customers in country) but if they have colleagues from other countries with them they need to speak English.
So in a typical meeting that might have people from France, Germany, Italy and Romania, it will be conducted in English.
I've assumed that it is the same for Thomas Cook because it is an international company.