SharePoint sometimes has separate language packs for several variants of closely related languages, like Brazilian and European Portuguese, or several varieties in the Serbo-Croatian family of languages. In other cases, you have to live with a different variant of your language, so for instance people in the UK must use the American flavour of English with American spelling and vocabulary, and Flemish-speaking Belgians must use Dutch, with all its differences in vocabulary.
For Persian, the language spoken in Iran, there is no direct support. However in SharePoint 2013 and 2016 and in SharePoint Online, you do have the option of selecting Dari. Dari is a language of Afghanistan, the native language of about 20% of the population. It is also known as Dari Persian, Afghan Persian, and simply Farsi. Although the spoken version sounds quite different from Iranian Persian spoken in Tehran, the written language is nearly the same. Download the SharePoint Dari Language Pack and use it instead of Persian.
For languages where SharePoint supports a slightly different version of the language that you want, PointFire gives you the option in a lot of cases to override specific text from SharePoint's language packs and use your different text instead.