This is displayed in the top header (yellow box) which lsits the "proiritized" languages. Anyway there's another setting (not displayed) that disallow other translations (this was discussed recently when someone created a page to translate ONLY in minority languages, exclusing all other major languages than the source English).
Experience shows that (espacially for minory languages that very few translators available, many of them not being very fluent in English) we need to always enable other languages thant English to help disambiguate the English source so that translators for minority languages can see translations proposed in other major languages and that are better reviewed, in order to provide better translations not just based on the English source but also on what they can better read and understand in another language.
I must confess that I see no reason to exclude any language (except old compatibility languages whose use is no longer recommanded, and which may be configured globally, only if there's other choices of language codes: those compatibility codes will finally be redirected to new ones or locked down).
And for the ~15 major languages of the world (that are used as lingua francas for regional or national interchanges) they should never be disabled (notably because they are already used as fallback languages for missing translations in minority languages). For a list of fallback languages (that should never be disabled), see for example what is implemented in Commons and MetaWiki; this is more than just English which is only the last-chance fallback ! The list of fallbacks was first implemtend as templates ,then as modules, and now as part of global wiki configuration (and part now of MediaWiki itself with its API).