Tanaka-san or Johnson-san.
There is a vast list of names for "client" used all over the world of Shadowrun.
While in North America "Mr." or "Mrs. Johnson" is the common name, other names are used in other parts of the Sixth World, such as:
Herr/Frau Schmidt - in Germany - i.e. the Allied German States
Herr Keller - in Switzerland
Herr or Doktor Nowak - in Austria
Mr. Smith or Mr. Jones - in Great Brittain
Mr. Tanaka - in Japan
Mr. Wu - in China (including Hong Kong)
Mr. Kim - in Korea
Mr. Khan - in Pakistan and Afghanistan
Monsieur Dupont - in France (outdated - in the 2070ies French
Johnsons use various pseudonyms like Martin, Leblanc, Beaumont,
Sinclair or Noël)
Pan Jonáa - in Czech Republic
(Herr) Kowalski - in Poland
Meneer Janssen - in the United Netherlands (VNL)
Señor Fernández - in Spain
Gospodin Nikogo, in Russia
A special case are the Johnsons of the German Saeder-Krupp Corporation, because they call themselves "Herr Brackhaus". - Sometimes this may even be Lofwyr himself in metahuman form...
Tanaka-san (English, Mr. Tanaka) is the official name used in the setting, as seen on Shadowrun 5E Core Rulebook (page 28):
Sure, in Japan he sometimes calls himself Mr. Tanaka, in the Allied German States he's Herr Schmidt, and in Hong Kong he might go by the name Mr. Wu
Johnson-san or John-san has been used unofficially by the community for Japanese "clients". But I don't recall it being used on any official sourcebook.
The German wiki also uses Meisho-san, which translate to something like "Mr. Famous" or "Mr. Important".