While Western languages are written left to right, others are written (broadly) right to left. Even in right to left languages, specific regions of text (such as numbers) are written left to right. The behaviour of ‘enclosing’ operators brackets adds additional complexity.
The rules governing exactly how layout should proceed when faced with a combination of left to right and right to left text are complex to say the least, but fortunately an algorithm has been published by the Unicode consortium to specify exactly how layout should proceed in any given circumstance (as ‘Technical Recommendation 9’).
MuPDF contains an implementation of this algorithm derived from the example code provided with this recommendation.