Bug when deleting the singular translation of a translation with plural

They need to preserve their plural variant delte.

E.g., if the English string to be translated is:

[
  0 => '1 apple',
  1 => '@count apples',
]

Let's take Polish, which has 3 plural variants. It will go as:

[
  0 => '1 jabłko',
  1 => '@count jabłka',
  2 => '@count jabłek'
]

If we delete the singular (index 0) the result should be:

[
  0 => '',
  1 => '@count jabłka',
  2 => '@count jabłek'
]

But if we delete instead the 1st plural form:

[
  0 => '1 jabłko',
  1 => '',
  2 => '@count jabłek'
]

... or the 2nd plural form

[
  0 => '1 jabłko',
  1 => '@count jabłka',
  2 => ''
]

This should be fixed in the form but also on Excel import, if case.