Excel 2011 - comma separators instead of periods

Just bought excel 2011 and now all of my documents insist on using commas instead of periods.

I tried to fix it and change the currency, tried changing the numbers to different formats, and tried writing my own custom numbers-- which worked until I had a number like 1.3 (where I had to write the dot) and it assumed it was text or rounded it up or down. When I've tried writing the numbers and using the comma the program rounds the number up or down. Also, the formulas only work if the numbers are whole (1, 3, 4, etc.). It doesn't seem to matter whether the numbers are a currency or a general number.

Please help?

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