If the folder is completely gone, you can find manual replacement guides on the Steam Community which provide links to the necessary .iwd and zone files.
Community-verified English language packs for MW2 (2009) can be found at the following locations: cod mw2 zone english folder link download link
If you are specifically missing .ff files like common.ff or localized_code_post_gfx_mp.ff , ensure your folder structure matches the official depot: .../Call of Duty Modern Warfare 2/zone/english/ If the folder is completely gone, you can