Other than updating to LR 5.7.1 which might have fixed the issue, I'm not sure what to tell you, and now that LR 6 is out LR 5 won't receive any updates so however it acts, now, is it.
Can you export one of your green photos as a DNG, upload it to www.dropbox.com (or similar site) and post a public share link to it, here? I'd like to see if LR 6/CC 2015 still has the issue and have the English version so cannot reproduce the issue, myself.
We have verified this issue in the LR6.5 latest build on Win 10/Mac 10.11 DEU OS and this issue has not reproduced.
1.Import pictures then take the label of those pictures as green.
2.Input "grün" in Metadata(say Titel&Creator) 3.Export those pcitures.
4.Pictures can export successfully.See the screenshot.