Learn Farsi: من ژاپنی بلد نیستم.

It is created by taking the noun ژاپن (Japan) and adding the suffix -ی (-i). This is a very common way in Farsi to make a relational adjective, which here turns the name of a country into the word for its language or nationality.

Why does the sentence use بلد نیستم (I do not know) instead of نمی‌دانم?

In Farsi, you use بلد بودن (to know or be familiar with) for acquired skills, abilities, and languages. You use دانستن (which gives the negative نمی‌دانم) only for knowing facts or pieces of information. Since speaking Japanese is a skill, بلد is the right word here.

Why is there no word for 'language' here, like زبان ژاپنی?

You absolutely can say زبان ژاپنی (the Japanese language), but it is very common to drop the word زبان (language) in everyday speech. The adjective ژاپنی on its own clearly means the language when paired with a skill-based verb like بلد بودن.

What is the literal translation of بلد نیستم?

نیستم is the negative form of هستم (I am), so it means 'I am not'. Because بلد means 'familiar' or 'skilled', the literal translation of the phrase is 'I am not skilled [in] Japanese'. This helps explain why Farsi uses the 'to be' verb here instead of a regular action verb!