Internationalisation and Localisation are often referred to as i18n and L10n. Once you know why, it seems so obvious. Originally developed at Digital Equipment Corporation in the 70s or 80s, Wikipedia claims that:
The capital L in L10n helps to distinguish it from the lowercase i in i18n.
I would also suggest that it has a lot to do with one of the most popular fonts available to programmers and computer users at the time: Courier (Wikipedia), designed by Howard Kettler (bio). As you can see from this picture, the 1 (one) and the lower case l (L) are easily mistaken: i18n v I18n; L10n v l10n