If you have been invited to a Japanese wedding and have no idea what to wear, how about renting an outfit?
There are many formal wear rental sites online where you can rent outfits for functions held in Japan.
After the shop has confirmed your payment, they will ship your order to your residence in Japan.
Wear the outfit to the wedding and send it back to the shop within the stated rental period (no cleaning needed!).
Rental period is in the form of ◯泊◯日where 泊 refers to the no. of nights and 日 refers to the number of days.
Postage fees are usually fully-borne by the shop.
Some shops have an insurance package option that provides insurance in the event that your rented outfit gets damaged e.g. someone spills wine onto you
I recommend adding the option as you would have to pay for any damages otherwise.

Ladies:
You can rent a full coordinate i.e. dress + bag + accessories, or you can just rent the dress by itself.
Dresses are categorized according to age group and the type of event e.g. wedding ceremony or graduation ceremony so you can easily pick an outfit appropriate for the occasion!
https://partydressstyle.jp/
https://onepiece-rental.net/
https://www.cariru.jp/
There are many rental sites as you can see, but Party Dress Rental is by far the most popular.

Gentlemen:
Rental sets usually include a shirt, bottoms, suit jacket, tie, belt and shoes.
https://rental.rakuten.co.jp/genre/101000/