The space opera series is indeed German. It was begun in 1961 by German authors K. H. Scheer and Walter Ernsting (aka Clark Dalton). Set 10 years in the future (in the far off world of 1971) it tells the story of the first manned moon landing - an American mission commanded by Major Perry Rhodan "ace astronaut of the United States Space Force." As The Custodian indicates, the astronauts find an alien ship on the moon, comanded by the wise, elderly alien scientist Khrest and the beautiful, intelligent and determined female commander Thora. Rhodan wins the trust and respect of the former and the love of the latter as things progress along the lines described above. The alien Arkonide race has succumbed to fatigue after many millenia in space, and the "young and energetic" Terrans become heirs to their dominion over the universe. Other characters included Atlan, a time-traveller from Arkon's glory days and Pucky the Mouse-Beaver (called Gucky in the German).

The series was published in the United States by Ace Books starting in 1969 with #1, Enterprise Stardust. Publication of the series ended in 1977 with an Ace Double style run containing #117 and #118 The Shadows Attack. There were several novels at the end of the run - 5 stories about the character "Atlan" and finally in January 1978 Perry Rhodan In the Center of the Galaxy, the last Perry Rhodan book published by Ace.

Oddly enough the popularity of Star Wars and its many imitators put paid to the "Peacelord of the Universe" in North America as audiences demanded a different style of entertainment, and as their homegrown science fiction options grew. Nonetheless, the German original is over 1800 installments long and AFAIK is still going strong.

Update: Dhericean tells me that the Germans are now at ~2028 (#2000 was published in Dec 1999). Also Forry Ackerman managed to publish up to #137 in the US (after Ace dropped it).
Update 2: gwidion says that in Brazil they were published until 1992, and got up to #537, translated into Portuguese by "Ediouro".

I have an almost complete set of the Ace novels, remnants of a misspent youth. I have a few duplicates, especially of the early series, and would be happy to trade the extra books with anyone who's interested.