To be "fluent" in Japanese means different things to different people. That said, one such criterion would certainly be the ability to function competently in the spoken and written language in the course of day-to-day life.
Take, for example, being able to negotiate the maze of 申込書 (mōshikomi-sho, application forms) and 質問書 (shitsumon-sho, questionnaires) with which we are regularly confronted. In my personal view, this delineates from being conversational or reasonably functional in the Japanese language to being truly proficient.
I think back to several decades ago, when local hospitals didn't even attempt to screen new non-Japanese patients in writing but used staff members to interview them verbally. On one memorable occasion when I went for an MRI scan, a Catholic nun clad in a habit actually asked me, 現在、妊娠されていますか？ (Genzai, ninshin sareteimasu ka?, Are you pregnant?) これは作り話じゃない (Kore wa tsukuribanashi ja nai, This isn't something I made up). The answer, obviously, was no, but 僕は赤面した (Boku wa sekimen shita, I blushed).