全文検索にPGroongaを使うように改造したので、日本語も検索できるようになりました。
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# Install PGroonga
sudo tee /etc/apt/sources.list.d/groonga.list <<'END'
deb [signed-by=/usr/share/keyrings/groonga-archive-keyring.gpg] https://packages.groonga.org/debian/ buster main
deb-src [signed-by=/usr/share/keyrings/groonga-archive-keyring.gpg] https://packages.groonga.org/debian/ buster main
END
sudo curl -o /usr/share/keyrings/groonga-archive-keyring.gpg https://packages.groonga.org/debian/groonga-archive-keyring.gpg
sudo apt update
sudo apt install postgresql-11-pgroonga

# Migrate Database
cd /opt/pleroma
sudo -Hu pleroma MIX_ENV=prod mix ecto.migrate --migrations-path priv/repo/optional_migrations/pgroonga_indexing/

# Configure
sudo tee -a /opt/pleroma/config/prod.secret.exs <<'END'
config :pleroma, :database, fts_index_type: :pgroonga
END