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# KEY   LABEL   TRANSLATION OUT OF DATE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.