Indeed the differences in some locales may completely change the native alphabet that the user is prepared to work with. Even the direction of the text flow may change in extreme circumstances. Moving around Europe certainly requires that different accented symbols be used in each country.
Fortunately the Unicode standard covers a great many of these national variants.
Typically, a localized implementation may allow the use of accented and national characters in identifier names but would still retain the English spelled keywords and punctuation characters.
Date and time formats tend to be arranged differently in each country. The United States and the United Kingdom commonly place the day number and month number the opposite way round. For day numbers above 12, this can be detected automatically but otherwise the context needs to be examined to decide which way round these values are.
Numeric formats may dictate a different punctuation symbol to be used for the decimal point and thousands separator. Confusingly, the comma and period characters may be used in both contexts. This means that a value could be misinterpreted and might yield a result that is several orders of magnitude in error when parsed.
Currency symbols will need some attention. The United States dollars symbol is in the lower 128 character set. The cents symbol is not. The UK pounds symbol is within the lowest 255 characters.
Be careful of the following:
Decimal points and thousands separators