I started learning English as a child, had a break of a few decades, and have now taken up my studies again
Well, it's a bit of a story and the only way to tell a story is to describe every element. There are three points all included in the example sentence.
It has been a long time since I first studied English. I am studying the language again to get my fluency back up to where it was.
When talking about a long period of time such as 'a few decades' it is very normal in English to just describe this as a 'long time'. There is no need to be specific on the length of time you haven't studied for.
The second part of this answer tells the listener that you are studying again and why.
"A few decades"（数十年）単位の長期間を表す場合、通常、英語では"a long time"と表します。実際に、何年勉強したか具体的な数字を言う必要はありません。
二つ目の文章は、英語学習を再開したこと、何故そうしたのか、を聞き手に伝える内容です。
It's been awhile since I've studied English, and I want to study it again.
It's been quite a long time since I've studied English, so I am studying again.
The last time I studied English was decades ago, so I'd like to start again.
When you want to express a long length in time, you can use the phrases: quite a while, long time, decades, ages, so long, forever, too long. These phrases indicate a long time has passed since the action was taken. You can use the phrase, "It's been ___" to measure the time that has elapsed. You can explain, "I studied English decades ago and I would like to start studying again." Many will be very impressed that you want to learn English again!
長い期間を表したいときには、次の語句が使えます。
quite a while, long time, decades, ages, so long, forever, too long.
これらの語句は、その行動を行ってから長い時間が経過していることを表します。
"It's been ___" は経過した時間の長さを伝えるときに使えます。
次のように説明できます。
"I studied English decades ago and I would like to start studying again."
（何十年も前に英語を勉強しましたが、また勉強したいと思っています）
英語をまた勉強したいと聞いてみんな感心すると思います！
I am studying English again after a few decades of not studying
I have returned to study English after having a few decades off from studying
A decade is a period of ten years so it is quite a long time and you might forget things if you started doing something then you would say to do something (again) , you could also say you have returned to doing something
A decade' は「10年間」です。かなり長い時間ですから、忘れてしまう事もあるかもしれません。
もう一度何かを始めることは、'Again'（もう一度）で表せます。あるいは、'I have returned to doing something' という言い方もできます。
It has been many years since I studied English so I need lots of patience.
I know my English is not that good but it has been decades since I studied it.
I am eager to learn English but please be patient with me since it's been a very long time since I studied it.
It has been many years since I studied English so I need lots of patience.
I know my English is not that good but it has been decades since I studied it.
I am eager to learn English but please be patient with me since it's been a very long time since I studied it.
I'm sorry if I am hard to understand, I am just now studying English again after decades of not knowing it.
Please be patient with me since it has been decades since I studied English.
I apologize for my bad English, I haven't studied it in years.
It has been many years since I studied English so I need lots of patience.（何年も英語を勉強していないので、我慢強くやらないといけません）
I know my English is not that good but it has been decades since I studied it.（そんなに英語がうまくないのは分かっています。でも何十年も勉強していないんです）
I am eager to learn English but please be patient with me since it's been a very long time since I studied it.（英語をすごく習いたいんですが、長い間勉強していないので、辛抱強くお願いします）
I'm sorry if I am hard to understand, I am just now studying English again after decades of not knowing it.（分かりにくかったらごめんなさい。今何十年ぶりに英語を勉強しています）
Please be patient with me since it has been decades since I studied English.（英語を勉強するのは何十年ぶりなので辛抱強くお願いします）
I apologize for my bad English, I haven't studied it in years.（下手くそな英語で申し訳ないです。何年も勉強していないので）
This is the first time I have studied English in forever.
I haven't studied English since I was a student.
It's been years since I studied English.
These statements are all quite natural and fluent to say in British English. We wouldn't generally use the term 'decades' as a time frame in this context, although it isn't wrong to say it. We use phrases like 'in forever' or 'in years' to represent a long period of time. If you want to be more specific then you could give the exact number of years it has been since you studied English, or the exact number of decades e.g. 'it has been two decades since I studied English' or 'I haven't studied English in two decades'.
Also, instead of saying 'studied' it is common to use the phrase 'had English lessons' or 'learnt English'.
これらの文はどれもイギリス英語では非常に自然な言い方です。
この文脈では普通、時間枠として 'decades' は使いません、間違ってはいませんが。'in forever' や 'in years' などのフレーズで長い期間を表します。
より具体的に言いたいなら、'years' あるいは 'decades' の数が言えます。
例えば：
'It has been two decades since I studied English'（20年英語を勉強していない）
'I haven't studied English in two decades'（20年英語を勉強していない）
また、'studied' の代わりに、'had English lessons' または 'learnt English' を使うのも一般的です。
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