What does I18N safe mean?

Solution 1:

I + (some 18 characters) + N = InternationalizatioN

I18N safe means that steps were taken during design and development that will facilitate Localization (L10N) at a later point.

Solution 2:

This is most often referred to a code or construct ready for I18N - i.e easily supported by common I18N techniques. For instance, the following is ready:

printf(loadResourceString("Result is %s"), result);

while the following is not:

printf("Result is " + result);

because the word order may vary in different languages. Unicode support, international date-time formatting and the like also qualify.

EDIT: added loadResourceString to make an example close to real life.

Solution 3:

i18n means internationalization => i (18 letters) n. Code that's marked as i18n safe would be code that correctly handles non-ASCII character data (e.g. Unicode).