I'd like that the locale definition for en_DE to be included in glibc. The en_DE (and other en_XX variants) have been added to the CLDR (Common Locale Data Repository) database about 2 years ago as standard locale definition: http://unicode.org/cldr/trac/changeset/11969 (as a result of http://unicode.org/cldr/trac/ticket/8642 )
Isn't it sufficient to introduce en_EU (bug 22473) and thus avoid inflation of en_XX locales? Of course for every xx language and YY country we can find people in YY country who can speak xx language but we are not going to add xx_YY pairs for every single language/country pair. This is not an ultimate decision but I suggest to close this report as wontfix.
The proposal for en_EU doesn't work unfortunately as each European country has different local formats. See the specific bug report for more details. As maintainer of en_NL, hereby me support in favor of adding en_DE.