It's a really fascinating topic, but one thing to remember, a country can't dictate to another who is considered a citizen. If Sakai never followed through with his renunciation, the US would consider him a citizen. That might cost him his Japanese citizenship, but the US isn't a party to that action.
Long story short, there's no real way for them to enforce this and plenty of people have dual US-Japanese citizenship, for instance. It depends on the particular consulate or embassy, but in general they aren't trying to aggressively crack down or anything.
Gotoku Sakai with the VfB equalizer at Hoffenheim as his drive from right hits defender and catches Baumann's near postHoffenheim 1 VfB 1— FOX Sports Hub (@FOXSportsHub) February 14, 2015 @bungadiri @dark knight -- Let's update the title to "Gotoku Sakai at VfB Stuttgart."
Gotoku Goto Hamburg? Der Hamburger SV kann nach mehreren Wochen der Marktsondierung die erste Verpflichtung für die neue Saison vermelden. Gotoku Sakai wird vom VfB Stuttgart zu den Hansestädtern wechseln, sofern auf der Zielgeraden des Transferprozesses nichts Unvorhergesehenes mehr passiert. http://www.ndr.de/sport/fussball/HSV-Auftakt-mit-Talenten-aber-Sakai-im-Anflug,hsv15140.html
Now that he's a teammate of Bobby Wood, we'll be noticing him a lot more: Gotoku Sakai returned to training today - A day earlier than scheduled. He is keen! #HSV pic.twitter.com/nkEivB6e7A— HSV News English (@HSVnewsEN) June 29, 2016
I thought I noticed him getting torched by Pulisic this past season (at 5:50). http://www.starsandstripesfc.com/20...-every-touch-christian-pulisic-had-vs-hamburg
extends his contract until 2020 at Hamburg. Fix! Unser Kapitän hält Wort: @sakai_go1123 verlängert seinen Vertrag bei uns für zwei weitere Jahre bis Sommer 2020. 📋 Zur Meldung: https://t.co/Xr8rkHsdWx#nurderHSV pic.twitter.com/g9LwxDkwjE— Hamburger SV (@HSV) May 30, 2018
moves back to Japan. Will he eventually move to the USA? As a Japanese-German-American, he's played in 2/3 countries. 【完全移籍加入のお知らせ】DF 酒井高徳選手がハンブルガーSV (ドイツ)より完全移籍加入することになりました。Welcome to Vissel, @sakai_go1123 !詳細はこちら👇https://t.co/HbyuvpYho5#visselkobe #ヴィッセル神戸 pic.twitter.com/VRoKqd0OYb— ヴィッセル神戸 (@visselkobe) August 14, 2019 in all honesty he was poor for HSV in their last 1BL season when they got relegated and poor last season in 2BL. Ex captain too and he got tons of hate from fans. makes sense too Kobe has a German coach in Fink and will be teamates to some Legends like Podolski, Villa, Iniesta and guy like Vermaelen.
Apologies for not looking back through this thread to see if it had already been mentioned, but it seemed mildly interesting that he has a younger brother who is also a high-level player, a '92 named Noriyoshi. However, he wasn't born in the U.S., so assuming their parents are the same, this is just a footnote.
They also have Goson, a '96 who has been toiling in the 4th tier in Germany for the last few years. Their eldest brother Gohi is apparently USMNT-eligible, but opted for a career as a heavyweight judoka instead.