I think I almost have all the clues nailed down on this one, but am just missing a little something towards the end... The general idea behind this riddle is that:
Each stanza contains clues to words which conjure up ideas of REPETITION (hence the title: "Let's hear it once again").
If points were meals on Stack Exchange,
You'd do this, though it be strange.
If 'points were meals' you would REP EAT ('rep' = reputation, the currency of Stack Exchange).
It's in French when you're all viewing,
Too much boredom will be its undoing.
'When you're all viewing' - i.e. when you're part of an audience at a show - if you wish the act to be repeated, you would all shout "ENCORE", a French word meaning 'again'. ('Too much boredom' would not result in the audience shouting this - nobody wants to sit through something tedious a second time - hence it would 'be its undoing'...)
To check what you see, if you find me:
Pisan king, German river, instability.
There are two words hidden in this stanza. The word 'instability' clues that we are looking for anagrams connected to the phrases that go before it:
1. (Least sure of this one) Pisan kings of Greek mythology include Pelops and Oenomaus. Either of these could be described from their actions as an EGOMANIAC - a word which anagrams to COME AGAIN.
2. There are two rivers called the Oder which run through or along the border of Germany - this anagrams (in fact, reverses) to REDO.