In the international board section, only the following languages are allowed: English, Bulgarian, Danish, Dutch, Finnish, Lithuanian, Swedish, Slovenian, Slovak and Mandarin.
Unfortunately, French is not one of them. Closing the thread for this reason.
The good news is that French speakers have their own dedicated section.
I'm linking it for you here. š Forum Francophone
Assuming the happy paths, where full faith is available:
For instance, when you have hephaistos, the cooldown is 168 hours and the duration is 24 hours. Let's say you have cooldown enabled:
You then round up to the next number, so it would be 6.
For Demeter that would be:
So five is the answer you are looking for.
In case you are curious to take it one step further and calculate what the cooldown would be with less temples (or with incomplete faith levels), the calculation is relatively simple:
Let's take the example Hephaistos. It has 168 hours of cooldown.
Let's say I only have one temple and on that island, maximum faith is available. I do not have theocracy.
Then the calculation would be
Let's say I have two temples and theocracy enabled:
You could apply this formula until your cooldown is equal or lower than the duration of the wonder.
For instance, when you have six hephaistos temples and theocracy enabled, the values would be as follows:
Since the cooldown is lower than the duration of the wonder, the cooldown is said to be zero.
The exact same formula works for the other miracles.
To answer your question about demeter:
The effect is 1,5 day and the cooldown is 9 days.
You could do it with five temples if you have theocracy, because:
Which is below the working duration of 36 hours.
Het toont ongedierte bij een stad naar keuze. Meer dan een puur visueel effect is het niet.
Het verdwijnt na de aangegeven periode of indien de eigenaar 50 000 goud neertelt om zijn stad voor bepaalde tijd te beschermen.
Echt nuttig is de feature niet. Al kan ik me wel inbeelden dat het een uitlaatklep is wanneer de spanningen hoog oplopen in een oorlog. 
I can think three possibilities:
Probeer eens het volgende:
De vriendschapsbootjes die je momenteel ziet bij het overzicht is enkel voor bestaande connecties.
Spelers hebben niet meer de mogelijkheid om dit toe te voegen.
Jammer, maar het is niet anders.
If you like playing internationally, you may want to consider looking at either the UK or the US-communities.
Not sure which servers are vibrant though as I only play actively on one server.
The armour will be directly subtracted from the damage.
Let's say that in this case you would have catapults that are fully upgraded and you are not using Hephaistos (for simplicity stake). Your catapult would inflict 133 + 3 = 136 damage.
The armour of a wall piece is directly subtracted from that amount
136 damage - 40 armor = 96 effective damage
You would need 600 HP / 96 = 6.25 catapults to breach a wall piece.
Always round it up, so 7 would be the minimum for non-hephaistos attacks in this case. You should be able to breach 3 wall pieces with the amount of catapults you have available. But if nothing is behind the wall, breaking just one piece is more than enough.
It's a trade-off as upgrading a wall to maximum level would cost you 43 million resources per town. So is it worth it? That's up to you.
You could make the argument that even with a maxed wall of level 48 it is still possible to breach a wall piece in a single round. With all upgrades and hephaistos activated, you can even do that as little as 18 mortars. If you wouldn't have any units on the field to counter the invasion, you could lose the battle in the first round. Is a wall useless in such a case? Perhaps. But deciding a wall is not worth it based on that alone, is making you miss the point.
A wall should be considered a tool to slow down an attack, to buy you time. If it would slow down the attack two-three rounds (or even more) to break down the full wall, your attacker will need much more time to win the fight. This is useful as you will have more time to notice the attack and react accordingly. For instance, you could build some minimal troops on the go, forcing your attack to break down more of the wall or slow down the attack further down with balloons. The value of a very high wall is that it takes more time to break it down.
Your "claim" about the steam giants also doesn't match my calculations.
A stream giant has 184 HP and 3 armour points. A maxed out wall of level 48 does 192 damage.
Consider the steam giant can be upgraded via the workshop (3 base defence + 3 upgrade) and may be temporary improved using Hephaistos (+2 armour), these units would have a defence of 8.
Well, look at that: the wall does exactly 192 damage. Minus 8 defence of the steam giant, the wall does precisely 184 HP damage on the steam giant and kills one steam giant. You wouldn't be able to pull that off with a level 40 wall. Only a level 48 wall could pull this off.
Precision is especially useful in this because it indicates how the damage is divided across an enemy line if a single hit is not an instant kill. 90% of the damage would go directly to the attacked unit (which will not die the first round) and the remaining 10% would be divided over the rest of the pack. A maxed out wall would kill as much steam giants as wall parts remain when the wall can do its part. A lower level would not kill one in the first round immediately. An attacker could wave steam giants and minimize the damage.
Is this worth investing 43 million goods in the wall? That's a different question. Few people will attack with steam giants or wave it. And you will likely have other units behind your wall, weakening the steam giant before the wall comes into play. If the unit would have for instance 75% of its HP left, it might be more vulnerable for instant death with the 90% precision of the wall.
