This is actually turning out better than I expected. And one of the things that I have discovered is that the Translate Interface interface (as seen below) is the key. But also, that you don’t have to have everything listed here translated for most sites. For example, i have found that about 33% of my Views being translated, as it shows here, is actually about right for my site. Much of what isn’t translated are system areas that my members will never see.
So, go through the interface from the perspective of the alt language user and find the text strings that need to be changed, do a Find on them in the interface, and then change them. Much will be case by case. But be flexible; you may decide to change more than you need at the time so that the future stuff will already be in place.