Short answer
It reads what gets posted in a LINE group and posts it back in every language that group has set up — plain text, voice notes, and photos with writing in them. There is nothing to open and no second app: the translation arrives in the chat, under the message it belongs to.
The part that is easy to miss is that it works per group and per person at the same time. A group sets the languages it publishes in, so a Taipei–Osaka–Vancouver family chat can carry three versions of every message. Separately, each member sets the language PlumiBot speaks to them in — menus, buttons and confirmations, in their own language, without changing anything for anyone else.
Not all of it is free, and it is worth knowing which half is which. Text translation, the two games, the digest and stats are on every plan, the free one included. Reading a voice note, a photo or a shared link runs a second, larger model before the translation even starts — a photo costs roughly thirty times what a line of text does — so those need a paid plan. The alternative would be charging people who only send text for the photos everyone else sends.
PlumiBot