Added plural translation fallback

Some languages like Chinese don't have plural forms and the translations
didn't repeat the same translation for each form.

To prevent JS issues in such case, the translation logic will fall back
with the basic form when no plural translation is available.

Fixed #6427 

Tested file summary with Simplified Chinese, Taiwanese, French and English to verify that plural forms are still computed correctly.

Please review @DeepDiver1975 @schiesbn @karlitschek