All official phpBB extensions have to be located within ext/phpbb/.
And if you ever download an extension and don't know where to put it, look at the composer.json and look at the first line. So taking this extension as an example, this will tell you what directory it should be in: composer.json
CommunautéEzCom « Traductions d’extensions & styles pour phpBB 3.1.x & 3.2.x ».
Lists of all extensions identified for phpBB 3.1.x & 3.2.x.
Lists of all styles identified for phpBB 3.1.x & 3.2.x.
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