Add a way to choose between nl_NL and nl_BE

[samuelsov]

Overview

Support for nl_BE vs nl_NL as the locale was added recently.

Before

When nl is used in Drupal (or other CMS), the default inherited locale in CiviCRM is nl_BE.
It used to be nl_NL.

After

Default is nl_NL as before but it's possible to add a setting CIVICRM_LANGUAGE_MAPPING_NL to change this.

[civibot]

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[mlutfy]

Looks OK to me, but will avoid a same-shop merge.

[demeritcowboy]

👍

Note the code comment just above says to update templates/CRM/common/civicrm.settings.php.template too.

I'll update the wiki.