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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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
Happy is this/that king who all citizens love