Here's the list of strings English.lng and Italian.lng have in common, though some should obviously be the same:
STR_ACC Acc>
STR_ANKARA Ankara
STR_ANYTIMEPSITRAINING Psionic training at any time
STR_ANYTIMEPSITRAINING_DESC You can at any time send soldiers into (or removed from) psionic training. Remember, initial training takes from 30 to 60 days.
STR_APR Apr
STR_AREA_ Area>
STR_ARROWS ARROWS
STR_AUSTRALASIA Australasia
STR_AUSTRALIA AUSTRALIA
STR_AUTOSAVE Autosave mode
STR_AUTOSAVE_DESC 0 - Disabled. 1 - Quick save and quick load. 2 - Autosave only at 00:00. All save and load buttons is disabled. 3 - Same as previous, as well autosave on exit of the game.
STR_BANGKOK Bangkok
STR_BASE BASE
STR_BASE_ Base>
STR_BASE_UC_ BASE>
STR_BATTLESCAPE Battlescape
STR_BEIJING Beijing
STR_BOMBAY Bombay
STR_BRASILIA Brasilia
STR_BUDAPEST Budapest
STR_BUENOS_AIRES Buenos Aires
STR_CAIRO Cairo
STR_CALCUTTA Calcutta
STR_CANADA CANADA
STR_CANBERRA Canberra
STR_CANCEL_PROJECT Cancel Project
STR_CARACAS Caracas
STR_CASABLANCA Casablanca
STR_CHICAGO Chicago
STR_CYDONIA CYDONIA
STR_DALLAS Dallas
STR_DELHI Delhi
STR_DRAG_SCROLL DRAG SCROLL
STR_ELERIUM_115 Elerium-115
STR_EXPEND_ALL_TIME_UNITS Expend all remaining Time Units
STR_FEB Feb
STR_FULL FULL
STR_GEOSCAPE Geoscape
STR_GEOSCAPE_UC GEOSCAPE
STR_HANGAR Hangar
STR_HONG_KONG Hong Kong
STR_INDIA INDIA
STR_IN_TRAINING In{NEWLINE}Training?
STR_KARACHI Karachi
STR_KINSHASA Kinshasa
STR_KM km
STR_LAGOS Lagos
STR_LIMA Lima
STR_LIVE_LEADER Leader
STR_LOS_ANGELES Los Angeles
STR_MADRID Madrid
STR_MANILA Manila
STR_MAR Mar
STR_MAX MAX>
STR_MELBOURNE Melbourne
STR_MONTREAL Montreal
STR_MORALE MORALE
STR_NAIROBI Nairobi
STR_NEW_SAVED_GAME <NEW SAVED GAME>
STR_NEW_YORK New York
STR_NIGERIA NIGERIA
STR_NO NO
STR_NOT_ENOUGH_SPACE Not Enough Space!
STR_NOV Nov
STR_NOVOSIBIRSK Novosibirsk
STR_OK OK
STR_OPENXCOM OpenXcom
STR_PERTH Perth
STR_PLAYINTRO Play intro
STR_PRETORIA Pretoria
STR_PSI_AMP Psi-Amp
STR_PSI_LAB Psi-Lab
STR_QUICK_LOAD Quick Load
STR_QUICK_SAVE Quick Save
STR_RATING_OK OK
STR_RESEARCHEDITEMSWILLSPENT Researched items will be spent
STR_RESEARCHEDITEMSWILLSPENT_DESC Researched items are removed from your stores and will be disassembled. After "researching" living aliens, the body will be returned to the based stores.
STR_RESERVE_TUS_FOR_KNEEL Reserve Time Units for kneeling
STR_RIO_DE_JANEIRO Rio De Janeiro
STR_RUSSIA RUSSIA
STR_S s
STR_SACK SACK
STR_SANTIAGO Santiago
STR_SAVE_VOXEL_VIEW Save First-Person Screenshot
STR_SCREENSHOT Screenshot
STR_SEOUL Seoul
STR_SIBERIA Siberia
STR_SINGAPORE Singapore
STR_TANK Tank
STR_TIME_UNITS_RESERVED_FOR_KNEELING Time Units reserved for Kneeling
STR_TOKYO Tokyo
STR_TUS_RESERVED_FOR_KNEELING_AND_FIRING TU's reserved for Kneeling and Firing
STR_UFO UFO
STR_UFOPAEDIA UFOpedia
STR_UFOPAEDIA_UC UFOPEDIA
STR_UFO_ UFO-
STR_USA USA
STR_VANCOUVER Vancouver
STR_WASHINGTON Washington
STR_WEAPONSELFDESTRUCTION XCom 2012 item recovery
STR_WEAPONSELFDESTRUCTION_DESC Weapons carried by aliens will self destruct if their owner is killed.
STR_WELLINGTON Wellington
STR_X_COM X-COM
STR_ZOMBIE Zombie
for the interested, I got this list from the following command:
for fname in English Italian; do tr '\n' ' ' < $fname.lng | sed 's/STR_/\nSTR_/g' | sort >/tmp/$fname.lng.sorted; done; comm -1 -2 /tmp/{English,Italian}.lng.sorted