Use site.code instead of site.lang in interwiki.py
Use site.code instead of site.lang in interwiki.py

It can causes tons of bugs including problems in dump reading,
summary i18n, user-config.py checks, etc.

One of them: Bug: T92822

Change-Id: Id07e2e38efe35cf4cd8676b7ef2e8a27a6dbe296

hasteur/g13bot_tools_new,Darkdadaah/pywikibot-core,Darkdadaah/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,emijrp/pywikibot-core,h4ck3rm1k3/pywikibot-core,valhallasw/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,h4ck3rm1k3/pywikibot-core,happy5214/pywikibot-core,VcamX/pywikibot-core,icyflame/batman,xZise/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,darthbhyrava/pywikibot-local,TridevGuha/pywikibot-core,npdoty/pywikibot,trishnaguha/pywikibot-core,smalyshev/pywikibot-core,jayvdb/pywikibot-core,wikimedia/pywikibot-core,happy5214/pywikibot-core,jayvdb/pywikibot-core,npdoty/pywikibot