A little out of topic, but I would like to add on to what Fireflytech has mentioned.

I think Japanese and Korean language are pretty similar in term of grammar and word order, not all the time but pretty often. I guess the common similarity between all 3 CJK languages is perhaps the usage of Han/Chinese characters (kanji for Japanese and hanja for Korean). From the feedback of my friends, Han characters are something pretty hard for English / European students to swallow as these characters are actually a combinations of more than a hundred of radicals (character units). The learning and recognizing processes of these characters are highly cognitive where one really need to memorize the rules and patterns.

Sorry for the diversion.

To further you question. In my personal experience here in china I have noticed many Korean men and women are studying Chinese and vice versa (and chinese are studying Korean). This, as I discovered from asking both Chinese and Korean students, is because Korean and Chinese are similar in some ways. I'm not sure the ways they are similar as I only study Chinese but in my classes I find the Korean students have a better grasp of the chinese language then Native English / European students. Furthermore, I have also heard that Japanese is similar to both Korean and Chinese. This would make sense as they all three countries are very close together and share a long history.

I've heard once that English is part of the Romance Languages. it doesn't mean Romance as in flowers for your wife etc. it means its from Roman languages (ie Latin). almost all european languages except for two are these Romance Languages. I know one of the exceptions is Gaelic. If you study Etymology (word origin) you can see many english words are borrowed from other European words. for example

Zebra - from Italian, Spanish, Portuguese
Umbrella - Italian
Elephant - from old French
Voyage - from old French

So I guess the best answer to your question is any European language would be good for a native English Speaker to study as it does have similarities to english

if you realy interresant it you can try and you know may (experience is the best teachar)ı think you know spanish you can learn itallian and portuguese and french they are grammatical similiar langague but prounounce is diferent if you know english you can learn easly germany but gramaire a little different but there are very words similiar

       So strange: at first you wrote perfectly right <French tongue>, but then <Sorry, "a Frech tongue", I've made a grammatical mistake......>. And, on my opinion, in this meaning a word "language" <语> is more suitable than "tongue" which rather  <舌>, though I could mistake 'cos English is not my native.
       By the way, there is one more meaning of <舌>, the same to Russian as in Chinese "抓舌头", while English has "a prisoner for interrogation, or who will talk".