One of the most recurrent issue a PM has to face is Text Expansion/Contaction.
These two links show tables with percentages of text expansion/contraction for several language pairs (from and into english)
Text Contraction and Expansion in Translation | articles | Translation
Translation Text Expansion: How it affects Design -
I found it very useful when I find myself quoting projects.
One last thing! I knew about diferrent text expansion/contraction rates depending on the language pairs, but did you know this rate changes (quite significatively) within the same language pair, depending on the field (legal, medical, etc...)?
p.s: Such issue one one the biggest issue a DTPer has to face.