iOS will pick the one which is placed higher in the list of languages under:
Settings > General > International > Languages
For example consider the devices with language lists:
Device A
- French
- Italian
- English
- Dutch
- Turkish
Device A (currently set to French) will display your app in English. Because English is placed higher than Dutch.
Device B
- Japanese
- German
- French
- Dutch
- Turkish
- English
Device B (currently set to Japanese) will display your app in Dutch. Because Dutch is placed higher than English.
If user never changed language settings, default order of these languages depends on the targeted market of the device.