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OmegaT sounds like the main tool i was looking for.
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I feel I still ought to warn you from my own experience.
Yes, OmegaT is, /de facto/, the only serious tool of description available open-sourcely (and not tied-in into some specific context).
However, OmegaT's automation may covertly skew/corrupt the result when working with object having complicated structure, as I learned to my own detriment when trying to apply OT to the LO PO files set.
I believe the root reason for that is segment-based translation for which OT was initially created. This is from memory so there are no more details.
I understand you'll translate (monolithic) LO documents so you'll likely be okay, but still, beware.
Backup early etc. 
-Yury