@honormulan As long as the last semester hs language course (at the end of two years) is C or better you have fulfilled the two year language requirement. In other words, if the D was from a semester prior to the last semester, you have completed two years of high school and have fulfilled IGETC. But the last semester cannot be a C-.
That is because langauge is a validation subject, meaning the last C or better grade in a higher level trumps all earlier grades from lower-level courses (even Ds and NPs).