In Latin alphabet letters љ and њ are transliterated as lj and nj. Can't these letters also mean лj and нj ? Can in Serbian (and other Balcan languages) letter "j" be after "л" and "н"?
Sorry for English
The Latin counterparts to the Cyrillic љ and њ are supposed to be digraphs just as in the Cyrillic alphabet and they have their entries in Unicode (ǉ and ǌ), but they're not used in practice. What's done instead is they're written as two consecutive letters.
There are indeed many cases where a "j" can appear after an "l" or an "n" without it representing a single phoneme. That's why transliterating from Latin to Cyrillic is not entirely straightfoward for Serbo-Croatian unlike the other way around.