Kalau nak translate GOD to bahasa Malaysia, it would mean TUHAN
ALLAH is GOD for Islam. And the GOD for kristian is also ALLAH but.. that was during Nabi Isa and only until Nabi Muhamad reaches 40 years old when he was made the last prophet... from there the religion for ALLAH is only Islam seperti yg disebarkan oleh Nabi Muhammad. With Nabi Muhammad as the last prophet he preaches to people the Islam religion and AlQuran is the message from Allah tru him for all mankind. The bible today has been done and amended in many version which no longer valid when AlQuran is released and complete.
The same goes to Taurat and Zabur.. all 4 kitab is from the same Allah but what you should follow today is only AlQuran.
So when kristian want to replace tuhan with Allah, actually is not wrong BUT what is wrong is for them to still be kristian... if they believe god is ALLAH they should actually practice Islam.
This is the simple way how i understand it.