It should parse as:

    [
      ["x","\"x\"","","x\nx","x"],
      ["y","","","",123]
    ]

It's valid CSV, inasmuch as it's accepted and generated by a lot of tools and RFC 4180 ABNF:

    escaped = DQUOTE *(TEXTDATA / COMMA / CR / LF / 2DQUOTE) DQUOTE