Hello Jan S. and thank you for you enquiry.
You can try contacting the German Institute of Standards ("Deutsches Institut für Normung e.V.") and ask if an English translation of the Standard 31051 "Fundamentals of Maintenance" is available/in preparation. Some of the standards are already available in English, please follow this link to their website: http://www.din.de/cmd?languageid=en&wor ... =dinSearch
Hope this helps.