Well, this is now solved:
* As part of this custom form, I extract the values from custom user profile fields, many of which are defined with multilang values.
* These values need to be processed in order to ensure that the key is the english value, regardless of the language of the value.
* In doing this, I'm using force_current_language('en') to ensure I get the key in english, then doing force_current_language($prevlang) to put it back.
Something something something, force_current_language('') instead of ($prevlang), profit.