Translation:Let me know when you come to Germany.
as Ektoraksan said, we would assume the first part (gelince) refers to the same person as the second part (ver). I think nobody would ever say the sentence above if they were talking about a 3rd person. When there are 2 subjects, you don't omit the pronouns, definitely not both. We would then say "O Almanya'ya gelince (sen) bana haber ver"
I think that what you meant to ask was:
"Let me know when you'll be coming to Germany". ("Almanya'ya ne zaman geleceğini bana bildir").
This sentence indicates that you want to know when they'll be coming 'beforehand' (before they arrive in Germany).
The initial sentence, however, uses "gelince" (when you come) indicating that you want to to be notified once they arrive in Germany.
Nobody can know that without the context.
Seninle bir an önce konuşmam lazım. Almanya'ya gelince bana haber ver. (So it is "you" who come to Germany)
Babanı çok özledim, onu görmek isterim. Almanya'ya gelince bana haber ver. Böylece onunla buluşur ve onu görürüm. (Now it is "he" who comes to Germany in this context.)
Çok yorgunum, biraz uyuyacağım. Almanya'ya/İzmir'e/İstanbul'a gelince bana haber ver. (If you are on a bus or maybe on a plane you can also say this sentence to the one who sits next to you. So it is "I or we" who come to Germany.)