FFS::isContentEqual is used to implement string equality comparison for translations. This is important because translation string might differ even though the contents are equal. Example:
Translator writes:
The bunnies stole {{PLURAL:GETTEXT|one carrot|%{count} carrots}}.This might be exported as:
0: The bunnies stole one carrot.
1: The bunnies stole %{count} carrots.When the system then flattens then again, it produces:
{{PLURAL:GETTEXT|The bunnies stole one carrot.|The bunnies stole %{count} carrots.}}isContentEqual should return true for this string and what the translator wrote, to avoid FuzzyBot replacing the first form with the latter form after export-import roundtrip. That's easy to implement with GettextPlural::unFlatten.