We have a multilingual web-site which works OK except for the NL language: when I'm on a page and then switch to that language in the Sites tab, I am prompted to translate the page, but the Translate button is greyed out.
In admin mode, the language is enabled, and has a language prefix of 'nl'
In Edit mode, the languag is in the Available languages list as well.
Can anyone think of a reaosn why the Translate button is greyed out?
A bit unclear when you say that the language is in the "available languages list".
Common scenario for this would be that you have enabled a language in the admin -> config -> manage website languages but you haven't actually enabled the language for the site in edit mode. I mean you have not added the language as available content language for editors in the root or on the site start page. You need to enable the language for editors by for example going on the start page (use the all propertis view or if OPE mode scroll the page up so that you see the properties header), there is the tools button, click that and select "Language settings", under available languages click 'change' (if the page is inheriting the language settings from parent then you need to go to that page, one or more steps up in the page hierarchy). Anyways, select now in the dialog NL (mark the checkbox) and click save. Now the language should be availabe.
Back to the case where the translate button is disabled (before the above change), when you changed to NL language on the sites tab, if you hover your mouse cursot above the NL language option on the sites tab, does it say: The language is available, but not enabled for this site? That indicates that the above steps should solve your issue.
If the above doesn't fix your issue then you might have changed who is allowed to create, edit etc that language. By default Everyone is allowed to operate on all languages. If this is the case then you should go admin -> config -> manage website languages, select NL language and check the "Users/groups for creation and editing" option on the language page and add yourself to that group (if other than everyone) so that you can create that language content.
Thanks Antti - it was the final paragraph that sorted it for me: there were no users/groups set up for that particular language.