Add Serbian (Cyrillic) new language mapping
Add Serbian (Cyrillic) new language mapping

This language_mapping setting was requested by Crowdin support
team to make Crowdin understand that we want Serbian (Latin)
translations files locale code `sr-CS` and Serbian (Cyrillic) with
locale code `sr`, we were asked to add the same mappings to
Crowdin project settings at [1]

[1] `Project Settings -> General -> Export -> Language mappings`

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