In addition to your monthly repayment, you will pay an acquisition and a mortgage fee. Count 6% of the purchase price of the property for the registration fee and 1% for the transcription fee (both corresponding to your acquisition fee).
However, a tax credit of €20.000 € per person (€40.000 in case of joint declaration) is immediately deductible on registration and transcription fees. If the amount has not been fully used, the remaining balance can be viewed at https://guichet.public.lu/fr/publications/logement/credit-impot-guichet.html .
The tax credit can be used gradually, for other acquisitions, until it is fully consumed. In addition, you will pay for the bank fees and the notary fees that depend on your bank and your notary.