Not even sure if it can be useful to anyone; I've worked on this tool since I got an English copy of Rebellion, but I previously had an Italian copy of the very-first "vanilla" SOASE. I needed something to import the original/common strings in order to avoid having to translate the whole huge English.str file.
The tool is partially complete (can successfully load, save, group strings by ID/Value, apply the same translation to identical strings, take and revert snapshots, and even merge strings from other .str files), but some features are still missing (Find and Replace, notably). Other things require further optimization (the way differently-encoded files are handled, for example; but also performance, especially when merging different files).
It's entirely coded in C#, so it needs .Net Framework 4.0+ on your system to run (but if you have an up-to-date Windows 7 or above, you should already have it).
You can find the download link, the whole source-code, as well as the instructions at this link on Github.