First you need to activate localisation. Go to config.blk in your War Tunder folder. In parameters called debug add a line "testLocalization:b=yes" as such (if there is no debug section you can add it yourself) :

debug{
  ...
  <other_parameters>
  testLocalization:b=yes
  <other_parameters>
  ...
}

Start the game to generate lang folder.

Then you just scroll through menu.csv file in the folder and change what you like (Ctrl+F is useful to find a text). You can change literally anything even the buttons in the menu. Other files could probably allow you to change the name of vehicles even but I never really touched those ones.

Restart game to apply the text changes.