I think you’re reading the wrong comments.
Ignore the ones that talk about “what we say in English” or “how the English word series can be used” and focus on the ones that talk about what the German sentence means.
To reiterate: in German, die Serien unambiguously refers to multiple series.
Mizinamo I get that. It's just confusing for beginners. Please try and understand that some of Duo's sentences are not easy to fathom when you first meet them even if they're obvious once you know. I read all the comments "right" or "wrong". We're all just trying to learn here but thanks for your help.
"The series begins." and "Die Serie beginnt." are the correct and definitely used ways to say in American English and German, respectively, that one series is beginning.
"The series begin." and "Die Serien beginnen." are the correct and definitely used ways to say in American English and German, respectively, that more than one series is beginning.
The only correct translation of the plural German version is the plural English version.
Note: In English "series" is NOT a singular collective noun. It is a noun, whose singular and plural are spelled the same.
From The American Heritage Dictionary: When it has the singular sense of "one set," it takes the singular verb, even when series is followed by of and a plural noun: A series of lectures is scheduled. When it has the sense of ... [more than one] set, it takes a plural verb: Two series of lectures are scheduled: one for experts and one for laypeople.
Or any other (books, comic books, action figures, anything manufactured with a serial number, like computer parts, auto parts, etc.). If this is actually a partial sentence, it could be continued in a lot of different ways, i.e., this means literally that the "The series begin", but you can say "The [TV] series begin(s) today", "The series [of monitors] begin with the number 0A3".
The series is a collective noun
That is not correct. "series" is a countable noun and has both singular and plural forms.
For example, "The Big Bang Theory" is a television series.
"Young Sheldon" is another television series.
Together, they are two series. And both of those series might begin on the same day, and you might say: "The series are beginning!".
The German die Serien is unambiguously plural -- just one series would be eine Serie.
"The series (singular) I am watching is very interesting."
"The sieries (plural) I'm watching are very different. One is interesting, the other not so much."
In American English, "series" is a noun whose singular and plural are spelled and pronounced the same. For more info, see my comment above from 2 months ago.