Unable to translate existing nodes

I started implementing a multilanguage site in a new testsite. All worked as expected.
Today I installed and enabled the i18n modules in an existing website. The default language of this site is Dutch and up till now the site only has nodes in Dutch.
After making the required settings /admin/settings/language I tried to update the content types. But all multilanguage setting are disabled. I don't know how/where to find a setting to enable these options. All old nodes are visible in both the Dutch and English menu. The translate tab is not available. When I try to translate by modifying the URL into: node/999/translate, I get a "Access denied" error while logged in as user/1.
The same problem I experienced by the taxonomy. I solved this by first setting a term to English, after which I was able to change the language in the vocabulary.

I tried to find an issue like this, but couldn't find any.

I found the problem seems to be in this install function:

i18n_update_9() {
  // Update content type settings
  foreach (array_keys(node_get_types()) as $type) {
    if (variable_get('i18n_node_'. $type, 0)) {
      variable_set('language_content_type_'. $type, TRANSLATION_ENABLED);
    }
  }
....

where the variable not is set to TRANSLATION_ENABLED. Maybe it doesn't exist at that moment?
Even after update.php the all language_content_type_xxx variables were set to 0.