France
r/VosFinances is the France personal finance sub, you may also check r/EUpersonalFinance
French-Japanese convention
In place since 1995, a fiscal convention between France and Japan eases some of the dual taxation issues. As of 2021-05, the last update is from 2007.
See article 2 for the scope of the convention. Note this convention does not covers gift or inheritances, so there is a risk of double taxation.
Finding knowledgeable professional help :
The French Embassy provides a list of know lawyers, some with French and other with English capacities.
See comment here : https://www.reddit.com/r/JapanFinance/comments/mw0r7x/looking_for_professional_with_insight_into_france/gvjcmqn?utm_source=share&utm_medium=web2x&context=3