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HI, info is in below : https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack
Please let us know if this solved your issue or if need further assistance.
arman1122 said
how i can change my firefox language
If you want a single Firefox install but change the language of the UI then the mentioned Language Packs is the option. If you want to have a single Firefox install of one language then you can download it from www.mozilla.org/firefox/all/