nestačí nakopírovat .lng soubory a v nastavení si vybrat?
irfanview_lang_czech.zip ... čeština - ZIP balíček bez instalátoru

1) Find your IrfanView directory
2) Unzip all files from the language ZIP into the "Languages" sub-directory
3) Start IrfanView, go to menu: "Options->Change Language" and choose your language