Hello, those who have issues with _day_of_6 _day_of_5 and so on and didn't help also:

Step 2. Then find this in upgrade.php

$iLangID = (int)db_value( "SELECT `ID` FROM `LocalizationLanguages` WHERE `Name`='English'"

Replace the word 'English' with 'en' (or the abrevation for your language pack.

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Then like me you have own language compiled. You can try to
use other string aka not 'en' put 'rumenia* or whatever named u did give when you first time compiled your language.

Also see more you can directly add new lines to your language file.
Those lines you can find upgrade.php file aka array part.

But don't know.. if later when you recompile they will be added
again.