What's the difference between "langer" and "länger"? Google Translate returns "longer" for both?
We are talking about the adjective lang here.
Notice that the initial letter of german adjectives is usually lowercase, though there are exceptions, which I won't go into now.
The form langer is simply a form of strong declension of lang. It occurs in the following situations:
- nominative singular for masculine genus
„Es war ein langer Tag.“
("It was a long day.")
- genitive singular for feminine genus
„Langer Rede kurzer Sinn ...“
("To make a long story short ...")
- dative singular for feminine genus
„Sie lief stets mit langer Hose herum.“
("She always walked around with long trousers.")
- genitive plural for all genera
„Ich bin kein Freund langer Abende.“
("I am not a friend of long evenings.")
The form länger is the predicatively used comparative of lang.
„Deine Haare sind länger als meine.“
("Your hair is longer than mine.")
Google translate is inferior to a decent dictionary, and there are many ones online which would have answered this question (which is therefore likely to be closed).
Langer is the masculine adjective, as in langer Tag (long day). Länger is the comparative as in
Das dauert noch länger. (This will take longer).
As a supplement to Björn Friedrich's answer, two possibilities where "Langer" would be written with a capital "l" like you wrote originally:
If there is a friend of yours who is significantly taller than you, you might greet him with
"Wie geht's, Langer?"
("what's up, long (=tall) one?")
And also "Langer" is a German family name, e.g. of former professional golf player Bernhard Langer