When installing in ie Dutch notice appears about LOCALE_NOT_CUSTOMIZED

Yes, looks like it (for now). As you know but others reading the issue might not, #1189184: OOP & PSR-0-ify gettext .po file parsing and generation is targeted to decouple locale from the gettext parser though, at which point this should not be an issue anymore. Currently they are hopelessly intertwined.

I think this should do it. I'm not sure we should elaborate on the fact that a better solution is being worked on at #1189184: OOP & PSR-0-ify gettext .po file parsing and generation, just get this simple fix in if we need this bug fixed ASAP and get moving on #1189184: OOP & PSR-0-ify gettext .po file parsing and generation instead :)

I think #1189184: OOP & PSR-0-ify gettext .po file parsing and generation is complex enough on its own. The installer does not have tests to cover multilingual installation, and we are to work to change that process anyway. It would be good to focus on finally solving #1189184: OOP & PSR-0-ify gettext .po file parsing and generation instead of finding new ways to extend its scope. :/

re: Tests, I think the main complication we encountered before with this is that we'd need to set the .po file directory to some custom directory *before* invoking install.php for it to display/recognize a list of languages, at which point there is no DB available yet for the site being tested. The only place where install.php seems to be invoked from tests is core/modules/simpletest/lib/Drupal/simpletest/Tests/SimpleTestTest.php, which does not really test the installer, it tests the simpletest browser. So sounds like writing tests for this would require #630446: Allow SimpleTest to test the non-interactive installer being fixed first. I'm not sure we want to postpone our fix getting in sooner though.

I think some manual testing of the patch would be useful and then we can get in the fix to advance work like #1189184: OOP & PSR-0-ify gettext .po file parsing and generation without waiting on changing the testing system.

The patch in #2 works for me.

@rvilar: can you explain what did you do and what did you experience? Could you reproduce the error? Could you reproduce that it was fixed by this patch? A little bit more background on what you did would be useful.

First of all, I downloaded the Catalan translation for Drupal core and located to sites/default/files/translations/install.ca.po

When trying to install Drupal in another language different from English, Catalan for example, the following errors comes in every page in the installation process:

Notice: Use of undefined constant LOCALE_NOT_CUSTOMIZED - assumed 'LOCALE_NOT_CUSTOMIZED' in _locale_import_read_po() (line 98 of /home/ramon/dev/drupal8/core/includes/gettext.inc).
Notice: Use of undefined constant LOCALE_NOT_CUSTOMIZED - assumed 'LOCALE_NOT_CUSTOMIZED' in _locale_import_one_string() (line 385 of /home/ramon/dev/drupal8/core/includes/gettext.inc).

When applying the patch from #2, the message disappears and everything works fine.

Thanks, that looks good. Given that we cannot do automated tests for this, I think this should be good to go now.

Ok, this looks reasonable as a stop-gap fix.

Courtesy of the great #d8mi sprint in #drupaldevdays, committed and pushed to 8.x! This is my first 8.x-only patch. Gulp. :)

Done, removing sprint tag. Thanks all!

Fix tag.