Is it permissible for an immigrant in France to combine the Dhuhr and Asr prayers while sitting in a public park during his lunch break, due to the lack of a designated prayer area at his workplace?

Prayer is not permissible while sitting for one who is able to stand, as standing is a pillar that does not fall except with an excuse, by consensus. As for combining two prayers, if there is an inability to perform each prayer at its appointed time, and there is hope that you may have an excuse for combining them, then some scholars have permitted it for a general need if separating the prayers would be difficult for the مكلف (person obligated by religious duties).