Ref T10688. When you have a translation like "I saw %s dog(s)", you can provide multiple variants in the translation file that are selected based on the argument's plurality or gender.

However, if you pass something which is not a number and not gendered, we can't use it to choose a variant (we have no idea which variant we should select for a phutil_tag() or arbitrary string).

An example of bad variant data is:

'%s touched %s %s time(s).' => array(
  array(
    '%s touched %s once.',
    '%s touched %s %s times.',
  )
)

This data intends to vary on the third parameter (the number of touches) but actually varies on the second parameter (the touched thing).

Unless that second parameter is a number or a person, this data is incorrect and tries to vary on a non-gendered, non-plural value.

The correct data uses an extra array to vary on the third parameter instead of the second parameter:

'%s touched %s %s time(s).' => array(
  array(
    array(
      '%s touched %s once.',
      '%s touched %s %s times.',     
    )
  )
)