tcl/tk already supports bi-directional text, but in order to make setup look correct for Hebrew/Arabic/etc. readers, some adjustments should be made:
1) any text should be aligned to the right side of it's container widget
2) buttons in any screen or dialog should appear in reverse order
3) Next/Back/Cancel buttons should be aligned to the left side of the window
4) in case of "label + input widget" pair, their order should be reversed (so when you read from right to left you see the label and then the input widget)
5) in setup type pane, the selector should be on the right side, and the description area - on the left
6) the components tree should be R2L too (just as if you look at it in the mirror)
7) any place containing icon + text on the right, should show icon on the right and text on the left side of it.
I think you get the idea.
If you think this is doable, I promise to submit Hebrew messages for InstallJammer as soon as the R2L support is ready, and do the testing of the feature.
In order to be able to add R2L languages with AddLanguage API call, there should be also some switch that will tell installer to go R2L mode (e.g. optional parameter in AddLanguage method, or virtual text RightToLeftMode as a switch).