Is there any word missing here? should I add the word wa 'は' after 'nihongo'
so in this sentence は will be added after 日本ご as the word is introduced, or am I confused
If the word you’re talking about has not been mentioned prior, like if you randomly mentioned Japanese just to say you like it, then you would use は. If someone asked you if you like Japanese, が would be the better choice to respond since the person who asked already introduced the topic of Japanese.
It's still understandable and is sometimes heard in INFORMAL daily conversation. For formal use, either が or は is added, depending on which part of the sentence you want to emphasize.
日本語 が すきです: Focusing on the part IN FRONT OF が (i.e.日本語). You like JAPANESE but not other else, especially if you are choosing one out of several options. 日本語 は すきです: Focusing on the part AFTER は (i.e.すきです). Regarding Japanese, you LIKE it rather than HATE it.