For this I use a compare tool like beyondcompare from scootersoft. Then I compare the original english to my language and replace missing strings with english strings.
The next step is to compare all string contain placeholders.
In the final step I go there to translate the english strings to my language.

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen