Complex sentence: The date of its foundation is considered to be 24 April, 1862, when a large number of immigrants, mostly from Switzerland, but with a significant number from Austria, Germany and France, settled in the area.
Split into simpler sentences: The date of its foundation is considered to be 24 April, 1862. This year marked the time when a large number of immigrants, mostly from Switzerland, but with a significant number from Austria, Germany and France, settled in the area.