SOL stories are overwhelmingly written in English but another site has a section for stories written in French (and may have other languages). I am currently reading two SOL stories which have conversations in other languages relevant to the plot, usually without translation. (One is a student going to Sweden and one a Dutchman talking to his family). I don't dare ask my wife to translate the second!
To what extent (and this depends on the languages involved - I realise that versions of Spanish are used in the US) should authors use other languages?