In the framework of the enhanced border controls between Germany and Luxembourg related to the Coronavirus (Covid-19), the Luxembourg government has issued a press release regarding a certificate in order to prove the working relationship with the employer:

https://me.gouvernement.lu/fr/actualites.gouvernement%2Bde%2Bactualites%2Btoutes_actualites%2Bcommuniques %2B2020%2B03-mars%2B16-bescheinigung-berufspendler.html