It is not dialect. When you use the "-a" ending you also have to use the feminine form of the pronoun: "di" rather than "din".
Alternatively it is correct (if somewhat unusual) to write "jenten din". "Jente" is one of a 100 or so nouns that almost all Norwegians agree should use the "-a"-ending. If you want to avoid it at all costs, you could also write "piken din", but that does have a somewhat old-fashioned ring to it.
In Norwegian Bokmål you can choose between a two gender system (common - "en, -en, -er, -ene" and neuter "et, -et") or a three gender system (masculine "en, -en", female "ei (en), -a, -er, -ene" and neuter "et, -et, -/-er, -ene/-a").
Among those who use the two gender system there are a 100 or so nouns that still get the "-a" ending among most users, examples are "jente" and "hytte". Even the Norwegian Academy for Language and Literature, the NGO that regulates the Riksmål standard use the "-a" ending in these words.
In the Bergen dialect the "-a" ending is not used in speech, but in all other parts of the west coast it is. Given that Bergen's share of the population is less than six per cent of the total, I think the statement that "almost all Norwegians" agree still stands.
This course uses the "-a" ending for these words in the "best" translations, though it accepts the "-en" ending as well since it is correct if unusual.