I've been to many places in Japan but not overseas(abroad, outside Japan).
旅行したことがある、行ったことがあるというのは I have been to, I have traveled で表せますね。
「海外はあまり行ったことがない」という文だけだと、
I have not been to many places overseas (abroad, outside Japan).
と言えますが、
I have been to many places in Japan と合わせるなら、重複する部分は省略できます。
I've traveled throughout Japan, but not overseas/abroad/outside Japan.
「日本ではいろいろなところを旅行した」 という部分は
I've traveled around Japan, ということもできます。
(travel around で「あちこちを旅行する」・throughout Japan は「日本中」）
海外はそれほど行ったことがないんだよね は、上記（前半）の反対ということですから、
but not overseas. (「だけど、海外はそうじゃない」）
なお、このoverseasは、この場合、副詞として使っていますので、その前には前置詞は不要です。
ご参考まで。
I know all about Japan but my experience abroad is limited
Limited = Restricted in size, amount, experience or extent; few, small, or short.
‘A limited number of places are available.’
‘The legislation has had a limited effect.’
To know all about something =Attributive Knowing, or appearing to know, all the details about something
"I know all about the life of Charles Dickens and his works. I studied his books at university and also read his biography."
All over = everywhere.
"There were bodies all over."
Limited =サイズ、量、経験または程度が制限されていること。 少数、小さいサイズ、または短いこと。
例：‘A limited number of places are available.’
(数に限りがあります)
‘The legislation has had a limited effect.’
（この法律は効果が限定されています。）
To know all about something =何かについての全ての詳細を知っている、または知っているように見えること。
例："I know all about the life of Charles Dickens and his works. I studied his books at university and also read his biography."
All over = いたる所
例："There were bodies all over."
（至る所に死骸がありました）
I have traveled a lot domestically in Japan, just not much abroad.
I've gone many places inside Japan, just not abroad.
"I have traveled a lot domestically in Japan, just not much abroad. is an expression used to explain that you have visited more places in your home country than foreign countries.
I have traveled a lot domestically in Japan, just not much abroad.
（日本国内は沢山旅行したけど、海外はあまりないんです）
これは、日本国内は色々な所へ両行しているけど、
海外はほとんど訪ねたことがありません、と言う事を説明しています。
I have been to so many places in Japan but very few places overseas.
I have traveled all over Japan but I still have to explore overseas.
Some people like travelling local, that is, they travel all over their country but find it very difficult to travel abroad. The reason may be financial or fear of the unknown, fear to explore uncharted territory. Travelling overseas requires more money than travelling locally. So, if you want to travel overseas, you have to save some money so that your trip should be enjoyable.
So, you may say:
I have been to so many places in Japan but very few places overseas.
or
I have traveled all over Japan but I still have to explore overseas.
地元、つまり自分の国を旅行するのが好きで、海外旅行がとても難しい人もいます。その理由として、経済的なことだったり、知らないところへ行くのがこわいなどあるかもしれません。海外旅行は自分の国を旅行するよりもお金がかかります。ですので、海外旅行をしたいなら、貯金をして楽しめるものにする必要があります。
I have been to so many places in Japan but very few places overseas.
日本の多くの場所に行ったことがあるけれど、海外はほとんどないです。
I have traveled all over Japan but I still have to explore overseas.
日本中を旅行したけれど、海外も旅行してみたい。
I've traveled to many places in Japan but not much overseas.
I've traveled to many places in Japan but not much abroad.
I've traveled a lot locally in Japan but not much overseas.
We can use the words, "overseas," and, "abroad," interchangeably to both talk about places that are outside of our country of origin. We can also refer to traveling, "locally," as a way to refer to our travels within our country.
日本 
イギリス 
アメリカ合衆国 
南アフリカ 
