The Answer is Blowing in the Wind
The answer to your question depends on some things:
1. How often will you do this?
If you'll be getting Excel files several times a day, you should just buy Office for Mac and install Excel.
2. How important is fixing the formatting issues?
If the spreadsheet is for your personal use, and you don't care about the formatting, stick with Numbers. If you'll modify these spreadsheets and send them to others who will judge you on the spreadsheet's look, then get Office.
3. How much interactivity is there?
If you'll modify these spreadsheets and send them back, and the recipients don't have Macs/Numbers, then get Office.
4. How much do you value your time?
If you're a poor college student who can't even afford beer and you like solving puzzles and you have no social life, get Open Office and do the translation thing you mentioned.
5. Do you want the respect of your peers?
If you want to become the guy that people go to to get their Mac problems solved, dig into Numbers (there are some videos on Apple's web site and other training) and figure out how to correct the formatting. However, see #4.
Good luck!