Complex sentence: Other Spanish-speaking countries call it "Península Antártica", among them Argentina, who also call it "Tierra de San Martín", and have more bases and personnel there than any other nation.
Split into simpler sentences: Other Spanish-speaking countries call it "Península Antártica", among them Argentina. It officially also refers to this as "Tierra de San Martín", and Argentina has more bases and personnel there than any other nation.