Ensure your original game files are unaltered. Altered files often cause patch failure.
Go to the dta folder and replace the existing language.pbo with the English version.
October 2024 (Retrospective Analysis)
Some community members created translation mods that replace text and voice files with English versions. These typically involve placing a "sound" folder and a titles.txt file in the mod folder.
Run the executable ( ArmA_Update_1_18.exe ) and point it toward your main ArmA: Armed Assault installation folder. How to Fix/Change Language to English