Can't be done. The LR 6 license will not work on other versions of LR.
You probably ought to try to find a license for Lightroom 5 via third-party vendors such as Amazon or B&H
No it won't work. You would need a LR5 serial number. You would have to pay to get to LR6 when you have a new system, but would qualify for upgrade pricing. The alternative is to subscribe and then you can update anytime at no extra cost.
Thanks for your reply. When I buy a Lightroom 5.7 licence from the UK (or US or Germany), can I install a Dutch version for Lightroom 5.7 with that licence? Or as other question, can you choose the language at installation?