as I understand it, i18n is the abbreviation for internationalization, and L10n is the abbreviation for localization (preferentially not l10n - the lower case L is ambiguous in many fonts). The 18 and 10 stand for the number of letters omitted. I have to say this is the laziest and least informative way to abbreviate a term I have ever encountered. I suspect it is not so much an abbreviation as a shibboleth.