The cell formulas were actually stored in Reverse Polish Notation with each function represented as a number. The actual ASCII you entered was never stored anywhere.
It's probably the most interesting and amusing file format detail I ever stumbled upon.
There are few obscure functions that have parameters and those parameters can come in home language - so they will stop working. Also there are some issues with charts.
In general Excel is made for corporations and people from different countries send each other files all the time.
However it would be nice to be able to dynamically switch language. There are few websites that translates formulas.
The name you type in - say - a German version is saved as a "function number" on file.
When the file is opened in - still say - an English version, these "function numbers" are rendered as the English names.