I'm sorry but we cannot reproduce this issue on our side, we have two languages and flags are displayed in both languages. 

Please try to upload another language pack from the forum and see if the issue persists

The Spanish translation works also, so their seems a problem with the Dutch version indeed..
I have translated all "flag" related words under section BASE, do I need to translate also other sections which hit the moderation section?

EDIT: sorry I see now that the keys starting with "moderation_" also needs to be translated.. I will check that first

Only the base, you need to find another Dutch lang pack and try again, as the one you have is corrupted

Did you do the translation yourself or use the pack?

I use this pack because this seems the latest version. The translation was not 100% complete  so I translated the part which have to be done.

The dutch translation works for the complete site (incl. plugins), but only the moderation part show no items which are flagged as you can see in the attachment in my first post. (also no untranslated text like base_flag_blablabla)

Okay, I merged the BASE part from the English translation into the Dutch version.. and now it works..  :)

the flags were available.. but when I "unflagged" 1 item... a success message appears.. en then all flags items are gone again (but when I switch to EN they are back again) ....

Ketkey, you said you have Spanish language as well, does this happen to the Spanish version too?

The Spanish version is working, but I found the following strange thing:

If I activate NL language only (SP and EN are disabled in admin), no moderation details are displayed (it shows: no items)

and here is the strange part:
If I activate NL + EN (or SP) language, and select NL as active language, the items in the moderation are fully displayed in the Dutch language..

I'm consider to serve temporary both NL + EN as language from the start of my portal because I do no have the time to translate a clone of English again. (I translated already a lot of the current NL file which now seems buggy..).

Thanks for your help so far :)

can you please export the NL lang pack and attach it here, probably I will be able to find the issue?

Sure, here it is

Ketkew, unfortunately I don't know what the reason of this issue is, I have passed it to our developers, unfortunately, they are very busy right now and will have time to look at the issue only the next week. I'll keep you updated. Thank you for your patience. 

No problem, thanks again for your support until now :)

The issue is that you don't have translation for the groups items

content_groups_labelcontent_group_label

you can see it in the group.xml file if you add translation between the value tags and then import the language the issue will be gone

there must be some errors for url formatting due to the missing values

Thanks Ross, I tested it and it worked!

I notice now that there are a couple of more problems in the moderation section due to non- or forgotten translations.. I've got some work to do..

Great, thanks for letting us know.