Попробуйте:
import org.apache.spark.sql.functions._
val stringify = udf((vs: Seq[String]) => vs match {
case null => null
case _ => s"""[${vs.mkString(",")}]"""
})
df.withColumn("ArrayOfString", stringify($"ArrayOfString")).write.csv(...)
или
import org.apache.spark.sql.Column
def stringify(c: Column) = concat(lit("["), concat_ws(",", c), lit("]"))
df.withColumn("ArrayOfString", stringify($"ArrayOfString")).write.csv(...)