Since we started with moving outputs of the international effort into real world, people tend more and more to switch from English at all.
On the other hand the i18n process is a constant work-in-progress and as such it will always lag behind default English localization which will most likely be the only one language kept up to date.
Since we used to release a properties files just with few translated keys, you can easily end up with a pretty malformed interface with few languages and missing values of a lot of keys (the ???key??? notation) like f.e. with current stk-1.4.1.
Thus it would be nice to introduce a super fallback language so the i18n chain will be like this: user's language -> configured fallback language -> English.