I just did the following on my MegaGlest 3.9.0 installation on Linux x86_64 (with German system locale):
./start_megaglest
-> Went to options menu, changed language to Gaelic (was: German)
-> Quit the game
./start_megaglest
-> Clicked on "New game", "Custom Game" (both in Gaelic)
-> Activated "translated techtrees" option (Gaelic)
-> Clicked on "Start game" (in Gaelic)
The game starts. I click on units (stickman, axe thrower, archer), structures (main teepee), resources (stone) - all descriptions are entirely in Gaelic. The only English word I come across is "Air" as part of "Air Milleadh").
When you do the same, do you see anything which is not in Gaelic? If not, what are your exact steps to end up with English texts?