From the KDE translations, I compiled statistics for text expansion, following the model presented by IBM on this page. So it will tell you for example that if you want to translate a text from English to French, which is between 11 and 20 characters long:

• if you want 50% of such texts to fit, you will need 24% of additional space (for an English text of 100 pixels, you will need 124 pixels)
• if you want 80%, you will need 57% of space
• for 90%: 77%
• for 95%: 98%
• for 99%: 150%

Sources and data are available. Please excuse me for the poor HTML design (written in HTML 5, looks like HTML 1.0 ☺), I did not put any effort in it (not my cup of tea), but at least it does the job.

I also wrote another version of the generator using the Go language. I was a little bit disappointed because:

• I did not get a formidable boost in performance (I should have, I have a 4-core CPU and Python can not fully take advantage of it because of its GIL). In fact, the file parsing part (the most time consuming one) gave similar performance, and the statistic production part was notably faster
• The GTK/Pango (I used to get text length in pixels) binding for Go was incomplete and boring to complete (even if Go has a really nice support to write C bindings). I also had some crashes (related to GObject reference counting).
• There is a port of Freetype written in Go, but the code to compute the length of a text was incomplete (it did not cover a given case)
• At the time I wrote this version, I thought about integrating Fontconfig directly in the program, but it would have taken more time that I wanted to spend on it.