Hm, is this a missing language in Yii framework too?

For that you should use the cldr setting:

$supportedLanguages['tet']['cldr'] = 'tl';

That should fix your issue. Let us know.

No, adding cldr in the surveytranslator_helper.php file doesn't solve the problem.

// Tetum
$supportedLanguages['tet']['description'] = gT('Tetum');
$supportedLanguages['tet']['nativedescription'] = 'Tetum';
$supportedLanguages['tet']['rtl'] = false;
$supportedLanguages['tet']['dateformat'] = 1;
$supportedLanguages['tet']['radixpoint'] = 1;
$supportedLanguages['tet']['cldr'] = 'tl'; 
$supportedLanguages['tet']['momentjs'] = 'tet';

I cannot reproduce.
Can you enable debug = 2 in application/config/config.php and try again, please? Paste the complete error message here.

In fact it's working with:
$supportedLanguages['tet']['cldr'] = 'tl';

Maybe a problem of cache when we did the test with that configuration.

Thanks a lot

Alright, no problem. Thanks for the feedback!
I added a small explanation to the wiki page for that parameter.