Several times within the last year, maybe two, I ventured into the depths of the interwebs in search of a monolingual Japanese dictionary app that could be used offline. What I found was JE-EJ dictionaries, 広辞苑第七版 for €89.99 and people in forums suggesting apps that wouldn't work offline. On my most recent desperate search I even came across a geoblocked dictionary app—to top it all off by a company deriving their name from the words "big" and "globe". m(
At this point I thought about just solving the problem myself. Looking at some dictionary websites, I found one where the entries were nicely enumerated from 1 to 297,199, so I wrote a small crawling script. In the meantime I used scholarly search engines to see if there happened to be some open data sets that could be used. I would've just thrown terms and definitions in an SQLite DB and installed some SQLite viewer on my phone.
In the end I found a better solution. One that's accessible and doesn't require technical skill—which is why I'm writing this. Watching this video (by this guy) I learned about EPWING, a JIS standardized format for digital dictionaries. Now, looking at aforementioned video you might get your hands on some such dictionaries. All you then need is an app to interface with them: I quickly found DroidWing, which I happily link here because the author also distributes a paid version where the only difference is the icon color. <3
So yeah, not bound to any service provider, platform independent, offline ... 国語辞典 case solved for good. :3
(This is just to feed the Google, in case someone else also looks for a solution to this: アンドロイド アプリ 国語辞典 国語 辞典 辞書 オフライン 無料.)