Yes it's a Japanese manga but pretty sure translated from Korean. It's a release from Random Fujoshis scans and they mostly do scanlation for Korean webtoons! (=・ω・=) this manga is probably from lezhin I think.
ahhh i see! that makes sense! thank youuuu (๑･◡･๑)
No problem! (⌒▽⌒)/
If you read the credit pages they already explained that they'll be keeping the Korean honorifics:
http://www.mangago.zone/read-manga/akai_ringo_no_nakami_jijou/uu/akai_ringo_no_nakami_jijou-chapter-1/3/
uhm i'm just a bit confused, why did the translator put korean words, "ahjumma" & "ahjussi"? isn't it a japanese manga or am i missing something?
neverthless the story is still great tho (๑˃ᴗ˂)