Well, yeah, McGrath, she could've been. But given the intensity of the emotion referred to in the song, it makes no sense to me to read something quite unnecessary (and unromantic) into the song. It's an inference not really called for, especially since soldiers did often meet, make love to, and even sometimes marry women in the countries where they are stationed.
Besides, the German lyric suggest they are both German, so why not assume they are high-school sweethearts who are torn apart when he is drafted? Happens all the time!
Genie