Wow! What an interesting question especially with grammar, language, and customs. I would think perspective comes in to play with that while surely that would be a field day for aficionados of etiquette. I am stumped while pondering especially since I am ignorant of languages foreign to English while am not an English grammar expert by any means.
Questions could be asked assuredly as there are belief systems that seemingly would not recognize God as the supreme deity, so why capitalize it other than respect. For instance would a Buddhist writer capitalize the word? I ponder perspective regarding the contrast presented with the author / writer and of audience.
A common occurrence is to capitalize Zeus (Odin) while one usually would say Zeus (Odin) is 'a' god while not saying Zeus is God. Yet, if one adheres to that as a belief system then is it not proper with the first person to capitalize it thus Zeus (Odin) is God? I would suppose with writing that may give impact to what is being authored presenting perspective with tense - first person or third person while also historical significance may enter.
hmmmmm . . . perplexing for me of least. I shall ponder that for adventures with writing in the future. Hopefully more will offer enlightenment regarding your question.