You need to set the commas to dots and the dots to commas so it is not a single find/replace, even if you use SGIII's "Find & Replace in Selection" shortcut. Using the shortcut (or the regular Find/Replace if there are no other commas or dots in the spreadsheet), you would need to change the dots to some other character, the commas to dots, then that other character to commas. After that you may need to set the format to number.
Or you can use the built-in tools. You said the language was correct, how about the region?
- Set the column to "text".
- Look to be sure the numbers are all correct. When importing numbers from one region into a spreadsheet set to another, some numbers might get misinterpreted. 12.345,67 might be treated as text (which is okay, we can fix that) but 12.000 or 12,000 might be twelve thousand when it was supposed to be twelve, or vice versa.
- Set the region (File->Advanced->Language & Region) to one that uses the format of the numbers in that column. Bolivia is one.
- Change the column to "number"
- Set the region to one that uses the 123,456.78 format. United States is one. Maybe the default System-Language is also one.