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Get the デジモンリンクス APK from Google Play or APKPure.
You just need to install the Japanese app. Bandai Namco publishes two separate apps: one global and the other for Japan. The Japanese version is called デジモンリンクス. Here it is in Google Play (the only official source of the app for Android, as far as I’m aware) and in App Store. I installed it from Google Play without any problems (it didn’t seem to be region-locked), but if you can’t do it or don’t have Google Play, it’s easily obtainable from alternative sources such as APKPure.
I assume that you can have both apps installed on the same device (and use them both) at the same time.
Of course, all texts are in Japanese, so you’ll need to know the language to understand texts like event descriptions. (I have no idea if you do, so I’ll proceed to give you some advice assuming you don’t; if you do, you don’t need any of this.) On the other hand, the UI is the same as in the global version, so if you know how that works already, you should have no problems; and event descriptions are somewhat translated over at Digimon Linkz Brasil (into Portuguese, but you can use the language selector in the top right corner to further translate that into English or another language through an automatic translator). The only nuance that might be of particular interest is that the word on the Agumon stamp in co-op & PvP, よろしく, is more versatile than the “Welcome” that the same stamp has in the global version. It can mean “greetings” equally well coming from a co-op host and from a co-op guest, and some players use it when they join a room and when anyone else joins a room they host, or at the start of a match; and it can also mean “please take care of [this]”, which I’ve seen someone use when their glass cannon Digimon died after doing the bulk of the work in a co-op quest to encourage the other players to finish off the remaining monsters. (It was met with “OK!”) I guess some words commonly used in co-op room descriptions may be useful as well: mostly のみ=only, 以上=at least, 解散=disband; on the other hand, 主=main, i. e. the host’s own Digimon, and [chip]です or [chip]持ちです also refers to the host’s own Digimon.
The game itself is pretty much the same as in the global version. More Digimon have been released, and you’ll see some in co-op and PvP. Advent quest drops are apparently different, from what I’ve seen in Reddit comments. (On Advent Hard, a luck drop in solo is always a fragment, while in multi I haven’t ever had a fragment co-op bonus drop.) Obviously, the current events and banners are different. I can’t think of any other differences, although admittedly I almost haven’t played the global version myself.