In case someone is wondering how musical pitch notation (A, B, C, etc. ) is expressed in Japanese, we use the old イロハ syllabary.
イ = A
ロ = B
ハ = C
ニ = D
長調（ちょうちょう） = Major
短調（たんちょう） = Minor
Thus, the 「ニ長調」 in OP's question translates to "D Major" in English.
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