If I install a german language module with Pelles C poide comes in german (not in english) without any /l switch. If I install Pelles C without the german language module poide comes in english.
Now the simple question: Where is the default language stored?
Maybe it is not stored at all. My speculation: first use appropriate language if /l switch is used and matching dll is installed. Second use system language if matching dll is installed.
Third (fall back) use english.