Nouns ending in -is are usually masculine.
Is japonais masculine or feminine?
lejaponais
The feminine form is japonaise
The rule behind it
94.1% of -is nouns are masculine
In context
Un Japonais ne ferait jamais une telle chose.
A Japanese would never do such a thing.
Y a-t-il des restaurants japonais près d'ici ?
Are there any Japanese restaurants near here?
Jane a fait de gros progrès en japonais.
Jane has made great progress in Japanese.