To cite x12 in publications use:

Kowarik A, Meraner A, Templ M, Schopfhauser D (2014). “Seasonal Adjustment with the R Packages x12 and x12GUI.” Journal of Statistical Software, 062, 1–21. doi:10.18637/jss.v062.i02.

Corresponding BibTeX entry:

    title = {Seasonal Adjustment with the {R} Packages {x12} and
    author = {Alexander Kowarik and Angelika Meraner and Matthias Templ
      and Daniel Schopfhauser},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {062},
    issue = {2},
    pages = {1--21},
    doi = {10.18637/jss.v062.i02},