Error when Switching Default Language in Multilingual Setup

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Hi all - I get the error below when I switch to Dutch language. What I did in order to get another language (and where I might be doing something wrong): Downloaded the language file from transifex.com, renamed to messages.mo, moved the file to /languages/nl_NL/LC_MESSAGES/ with Filezilla and then went to Dashboard to activate.

Hope this rings a bell, thanks already!

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Notice: No translation for the language 'nl_NL' available. in /customers/8/a/9/lijndiensten.com/httpd.www/updates/concrete5.6.1/concrete/libraries/3rdparty/Zend/Translate/Adapter.php on line 443 Warning: Cannot modify header information - headers already sent by (output started at /customers/8/a/9/lijndiensten.com/httpd.www/updates/concrete5.6.1/concrete/libraries/3rdparty/Zend/Translate/Adapter.php:443) in /customers/8/a/9/lijndiensten.com/httpd.www/updates/concrete5.6.1/concrete/core/libraries/view.php on line 919

LijnDiensten