When you run --validate-techtrees with the purgeunused option it will delete techtree localizations / translations. Techtree localizations / translations are legitimate files used by the game and should not be deleted.
Example output (kindly provided by Atze, redacted by me):
$ mk/linux/megaglest --ini-path=mk/linux/ --data-path=mk/linux/ --validate-techtrees=prax_0.5.2.2=purgeunused
====== Started Validation ======
Filtering techtrees and only looking for the following: prax_0.5.2.2
*NOTE All unused techtree files will be deleted!
---------------- Loading factions inside world ----------------
----------------------------------------------------------------
Checking techPath [/home/atze/.megaglest/techs/] techName [prax_0.5.2.2] total faction count = 2
...
Checking resources, count = 8
...
Errors were detected:
=====================
The Resource type [energy] is not used by any units in this techtree!
The Resource type [green] is not used by any units in this techtree!
The Resource type [kristall] is not used by any units in this techtree!
=====================
Found techtree filecount = 998, used = 673
Line ref: 2108, Warning, unused files were detected - START:
=====================
...
[/home/atze/.megaglest/techs/prax_0.5.2.2/lang/prax_0.5.2.2_english.lng]
[/home/atze/.megaglest/techs/prax_0.5.2.2/lang/prax_0.5.2.2_german.lng]
...
Purged 20.76 MB (325) in files
Line ref: 2142, Warning, unused files were detected - END:
Warning, duplicate files were detected - START:
...
Warning, duplicate files were detected - END:
----------------------------------------------------------------
====== Finished Validation ======