Need .stf example
Double check you're using tabs and not spaces and that the file is really saved as UTF-8 (you can use Notepad++ for example to check & force it).
Eh, the StackExchange's editor converts tabs to spaces. Check out http://pastebin.com/FEK7epN2 (raw data)
# Language: German
Language code: de
Type: Bilingual
------------------TRANSLATED-------------------
# KEY LABEL TRANSLATION OUT OF DATE
ButtonOrLink.Event.New_Event New Event Neues Ereignis -
ButtonOrLink.Opportunity.Add_Line_Items Add Line Items Artikelanlage -
# eyescream: skipped some
CustomLabel.PKB2_All_Fields_Required All fields are required. Alle Felder sind obligatorisch. -
CustomLabel.PKB2_Answers_Might_Help These answers might help immediately Diese Antworten könnten vielleicht helfen -
CustomLabel.PKB2_Back_To_Search_Page Back to search page Zurück zur Suchseite -
CustomLabel.PKB2_Cancel Cancel Abbrechen -
# eyescream: skipped some
CustomLabel.SC_Case_Information Case Information Beschreibungsinformationen -
CustomLabel.SC_New_Case New Case Neuer Kundenvorgang -
CustomLabel.SC_New_Find_Contact New / Find Contact Neuer Kontakt / Kontakt suchen -
CustomLabel.SC_Save_New Save & New Speichern & Neu -
------------------UNTRANSLATED-----------------
# KEY LABEL
ButtonOrLink.Order__c.Mass_edit Edit
ButtonOrLink.Order__c.Printable_view Printable view!
ButtonOrLink.Order__c.Quick_Email Quick Email
Not sure if you see it, I've "translated" one of last labels to "Printable view!" and imported whole file back to SF (I didn't zip anything, uploaded this one language directly):
So it ignores lines where the translation is blank or identical to your master language.