CS5 is delivered only with the language which is compatible to the serial number. Usual there was no way to change language.
It became possible with CC later.
First of all, thanks for your answers.
Unfortunately, the problem is still there after having tried Marc Autret's solution
As you can see on the following picture, the registry line was already regulated on the value 00000004 which corresponds to the french language.
I tried to turn the software in German for example (value 00000003) but nothing to do, the software is still in English. I did not have this problem on my other computer. Can't understand why.
Any other suggestions ?
Or the problem might come from the computer itself ?
As I wrote initially, I have not tried Marc Autret's solution myself. Moreover, the comments make clear that it is far from simple. Editing Windows' registry always entails risk, and the comments document various related issues, some of which fit your symptom InDesign defaulting to English. In particular, the user invoking InDesign must have read/write access to the key controlling the language setting, as summed up in Marc's comment #43 -- which might that explain why your original installation failed to honor your selection of French. Whether or no, I'd want to digest all the comments before starting down this path.