There are five allowable force selector lists:
* US
* German
* British
* Soviet
* French
From either the main Rulebook or an Army book/download (if available). Theatre selectors are not allowed as only some nations currently have them.
Regarding other nations such as the Japanese...if a Warlord selector list is available, they will be allowed. You can always run them as count-as and use another force selector list.
Speaking of which...
Count-as, proxy, substitutes: Count-as-models or substitutions are allowed as long as they are appropriate, consistent and do not confuse the opposing players ie. using a Games Workshop Land Raider for an armor car OR a set of Privateer Press' Protectorate of Menoth Exemplar Errants as US rangers would not be appropriate; while using a StuGIII as a substitute for a Marder III would be appropriate. Additionally, there are no company restrictions; with that said...Warlord is the sponsor and they have got the superb selection and quality. So check them out first