Translation:They say that German is a difficult language.
Coincidentally, they have the same comma rule in German.
„Man sagt, dass Deutsch eine schwere Sprache ist.“
One says, that German a hard language is.
(They say that German is a hard language)
It isn't just a writing rule, you actually pause when you're saying that out loud.
The phrase is "la germana lingvo" or "la lingvo de germanaj hemoj" but commonly you would drop the extra and just say "la germana". There isn't a word for each language, so we just shorten the common phrases. Note that "Esperanto" is the actual Esperanto name for the language though, so it doesn't need the la in front. You could technically say "la Esperanto lingvo", but it's never used by Esperantistoj.
This convention is similar to saying "ĝis" when the full phrase is "ĝis la rivido", also "dankon" or "saluton" when we mean mean "mi donas al vi dankon" or "mi donas al vi saluton". There are a few others of these in Esperanto, and you're fine to say the full phrases if you want to, but there isn't a need and it would mark you as komencanto.