This problem is not just in Japan but in other countries around the world too
Population decline is becoming a problem not just in Japan but in many countries around the world.
This problem is not just in Japan but～
この問題は日本だけでなく、～
in other countries around the world too
世界中の国でも
Population decline is becoming a problem not just in Japan but in many countries around the world.
少子高齢化は日本だけでもなく、世界中の多くの国でも問題になっています。
少子高齢化、を直訳するとちょっと長い表現になりますね。declining birthrate and aging populationになります。それでも通じますが、長いので大体population decline (人口減少)で表現します。
ご参考になれば幸いです！
The problems of an aging population and declining birthrates are not only problematic in Japan, but other countries around the world are facing these issues as well.
この場合、次のような言い方ができますよ。
ーThe problems of an aging population and declining birthrates are not only problematic in Japan, but other countries around the world are facing these issues as well.
「少子高齢化の問題は日本だけの問題でなく、世界のいろいろな国でも直面している問題でもあります」
an aging population and declining birthrates で「少子高齢化」を言えます。
ご参考まで！
アメリカ合衆国 
カナダ 
