。
Washlets themselves have barely spread outside of Japan.
Literal
Washlet itself [subject-が] Japan outside [possessive-の] countries [in-では] hardly spread-[negative-していません].
ウォシュレット is actually a TOTO brand name that has become genericized in Japan for any bidet toilet seat — a quintessentially Japanese bathroom fixture. 普及する 'to spread, to become widespread.' ほとんど…していません 'has hardly...' A fact that surprises many Japanese travelers abroad.