Bonsoir robin4002, merci pour cette réponse rapide.
J'ai installé JAVA, no problemo.
Clique PLAY une première fois : le download s'est arrêté sur "minecraft\sounds\music\game\end\end.ogg". Barre d'état à 100% mais bouton PLAY grisé. Après un long moment d'attente j'ai fermé le launcher et l'ai relancé.
Clique PLAY : le download recommence sur fastutil-7.1.0.jar (barre d'état à approximativement 90%) et ne semble pas aller jusqu'au bout et affiche à la place l'onglet Game Output avec les logs suivants :
[18:41:30] [Client thread/INFO]: Setting user: altumpilum07
Exception in thread "Client thread" [18:41:30] [Client thread/INFO]: [STDERR]: java.lang.NoClassDefFoundError: it/unimi/dsi/fastutil/ints/IntList
[18:41:30] [Client thread/INFO]: [STDERR]: at akh.a(SourceFile:193)
[18:41:30] [Client thread/INFO]: [STDERR]: at akh.a(SourceFile:109)
[18:41:30] [Client thread/INFO]: [STDERR]: at ni.c(SourceFile:477)
[18:41:30] [Client thread/INFO]: [STDERR]: at bib.<init>(SourceFile:394)
[18:41:30] [Client thread/INFO]: [STDERR]: at net.minecraft.client.main.Main.main(SourceFile:123)
[18:41:30] [Client thread/INFO]: [STDERR]: Caused by: java.lang.ClassNotFoundException: it.unimi.dsi.fastutil.ints.IntList
[18:41:30] [Client thread/INFO]: [STDERR]: at java.net.URLClassLoader.findClass(Unknown Source)
[18:41:30] [Client thread/INFO]: [STDERR]: at java.lang.ClassLoader.loadClass(Unknown Source)
[18:41:30] [Client thread/INFO]: [STDERR]: at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
[18:41:30] [Client thread/INFO]: [STDERR]: at java.lang.ClassLoader.loadClass(Unknown Source)
[18:41:30] [Client thread/INFO]: [STDERR]: ... 5 more
Java HotSpot(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
J'ai beau recliquer PLAY, on revient toujours là !
Voici ce qu'indiquent les 6 dernières lignes du Launcher Log :
[18:41:28 INFO]: Looking for orphaned versions to clean up...
[18:41:29 INFO]: Looking for old natives & assets to clean up...
[18:41:30 ERROR]: Game ended with bad state (exit code 1)
[18:41:30 INFO]: Ignoring visibility rule and showing launcher due to a game crash
[18:41:30 INFO]: Deleting C:\Users\LUC\AppData\Roaming\.minecraft\versions\1.12.2\1.12.2-natives-253225367389381
[18:41:31 WARN]: Couldn't delete C:\Users\LUC\AppData\Roaming\.minecraft\versions\1.12.2\1.12.2-natives-253225367389381 - scheduling for deletion upon exit
J'espère que je suis assez clair dans mes explications de noob !