"剑可以灭我们的国，但是可亡不了文化" seems ok, but it's better and clearer that you say：刀剑（或许）可以毁灭我们的国，但是可毁灭不了（我们的）文化！ I take it as " The sword can destroy our country, but can not destroy OUR culture." or: Our country(bodies) can be destroyed, but our culture(souls) will never be destroyed.
If you want to say something like "Death can not destroy culture", I guess "死神摧毁不了文化" will do. The whole sentence is like: 刀剑或许可以毁灭我们的国家，但死神是无法摧毁文化的。
The sword here means physical forces, and here it's better to be translated into "刀剑" or "武力”instead of "剑".