Hello, Using DroidSansFallback.ttf seems to be the smallest-sized way. I prefer this. :)
Or the stripped Android version of NanumGothic could be used instead of the full version in fonts-nanum. It also provides better looking Korean glyphs than the Droid font but it will still add ~1.25MB. https://android.googlesource.com/platform/external/naver-fonts/+/master/NanumGothic.ttf BTW, current Android (since 6.0) doesn't use NanumGothic but uses Noto Sans CJK. -- Changwoo
