Considering the original phrase:
The king who all citizens love is happy.
(Portuguese: Feliz é o rei a quem todos os cidadãos amam.)
Here is a proposed Latin translation:
Felix est rex is quem omnes cives amant
Is the pronoun "is" necessary?
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Sign up to join this communityNot sure where you got that is from. The original phrase, which seems to be from a 1845 book of Latin exercises, does not has it. For instance, see here.
In any case, is as pronoun means "he", or as demonstrative, "this/that", so maybe the phrase with the is could be translated as
Happy is he the king who all citizens love
or
Happy is this/that king who all citizens love
The best of Latin: the best of English: happy is the king; he (is), whom (quem) all citizens love. To omit "is" may be acceptable; but, the assertion becomes sloppy. Guys with the erudition of Cicero would not thank you.