Index local language - CCM_ACTIVE_LOCALE changes on form submission

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Hi,

I've found that Concrete5 switched to "CCM_ACTIVE_LOCALE en_US" when the page reloads to add the error message. Before submitting the the form it shows nl_NL correctly. It only does this on the index page.

My form errors are always English no matter what language I set for the website. Even on a multi language website the form error are English. Do I need to download a file or am I missing something?