Different regions use different formats for numbers, For instance, in the United States, a comma is used as a thousand separator (1,000) and a period as a decimal point (1.5), while in Germany it is the other way around (1.000 and 1,5).
Soulver needs to know which format to use to identify valid numbers. By default, it uses your Mac's system region settings. You can see and change your Mac's system region settings in the Language & Region preference pane.
You can select a custom region in Soulver's region preference pane. Existing sheets will be converted to the new region when you open them.