When – during a sabbatical leave – a young woman from Berlin falls in love with a man from Cape Town, she books two more flight tickets to find out – within a year – whether she should decide for her quite happy and successful life at home or what could become the love of her life.

I don't understand what is happening in your logline. I get the setup; your protagonist falls in love on vacation ... but I don't understand why she buys 2 plane tickets, and what the plan is to use them? Also, I'm not hooked by a film about a character "deciding" between one option or another. That's ultimately what all films are about, yes, but it's not interesting enough to sustain audience attention for 90 mins. Tell us what your protagonist's objective goal is after falling in love. What am I going to watch you try to achieve over the course of the film.

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"After falling in love while on vacation, a workaholic business woman takes a leave of absence to discover if it was true love or just a fling."
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I don't think hyphens are the best way to go in a logline. start with 'during a sabbatical (implies a leave), a young woman...
then try and cut out some of the second sentence. ...whether she should choose between her comfortable life at home...