As far as language in Cargolux is concerned then the only one that is essential is English. Whatever language you are interviewed in, at work all procedures are in English.
Whilst native German – in it’s different forms - speakers are in a majority in the company and bearing in mind that most Luxemburgers also speak it fluently, there is still a large minority who do not have it even to a basic conversational level (me for one).
The standard of spoken English varies from level 4 (very few) level 5 (majority) and level 6 (few). I have to complement my colleagues in that even if out socially with a group of native German speakers (or those with a different lingua franca) if there is someone who does not speak their language then the conversation will be in English.
Bearing in mind that all manuals, training and company communications are in English the higher the level of your spoken and written English the easier your life will be. Once on the line your Captain will be from one of 20+ nationalities some of who will have at least conversational German but some who wont. You may be faced with a native English, Danish, Icelandic, Norwegian, Swedish, French, Italian, Spanish, Dutch or Portuguese speaker (plus any I might have overlooked).
Good Luck.