production - Why does Star Trek use Celsius (SI system), not Fahrenheit (the imperial system)?

I think the "universal translator" takes care of it, as does the specialized translators used for ships log entries etc.

If Spock were to use a cultural reference in his Officer's Log, and speak of "a hundred twenty eight squelm" in FedStandard (which is decendent from and rendered as English in the show) the metadata would automatically note the standard value in kelvin, and later when a sulfer-breathing admeral from Sarr reads it, it will be in his native language with the value in kelvin and a footnote explaining that the author likened it to the desert mesa whatever blooms are triggered, with links. Or, it may show a notation mapping to the normalized clement range of the author, so he knows without distraction if that is supposed to be hot or bitter cold or whatever. In the case of a human reading, Fahrenheit might be one of the configurable options of the normalized clemency perception scale.

Since Starfleet is primarily founded and organized by Terran and Vulcan world governments, whose to say SI is the end-all/be-all of measurements? They might use Vulcan-based Interplanetary Standard units.