Japan has one of the lowest English proficiency in Asia.
In Asia, Japan is among the bottom countries for English proficiency.
Both of these examples let the listener know that in Asia, Japan has one of the lowest levels of English. 'Proficiency' is the level of skill, so Japan has a low proficiency in English. Both examples lets the person know Japan is low, but not the lowest.
どちらも、日本の英語力はアジア最低レベルであると伝える言い方です。
「Proficiency」は「スキルのレベル」を表します。「Japan has a low proficiency in English（日本は英語のレベルが低い）」と言えます。
どちらの例文も、日本の英語力が低いことを表しますが、一番低いとは言っていません。
Japanese have one of the lowest levels of English among Asian countries and especially many Japanese are not good at speaking English.
英語力という時に、特に"English skills"と言わなくても"English"だけで問題ないかと思います。
"one of the lowest levels of 〇〇 among □□" で「□□の中で〇〇の最低レベル」という表現になります。
"many Japanese"の後は複数形なので"are"が続きます。「会話が苦手」というと言語というより「コミュニケーションスキル」の話にそれてきてしまうかもしれないとおもったので、あくまでも英語のスキルの一つとして"speaking"ということにしてみました。
Japan has one of the lowest English Proficiency Index scores in Asia
Amongst Asian countries, Japan has one of the lowest levels of English proficiency
Japanese people have lower English language skills compared to other Asian countries.
The "Education First English Proficiency Index" ranks countries based on the English language skills of adults who have taken a specific English test. Japan was ranked 37th on this list. This means it has a low English proficiency compared to other countries.
「Education First English Proficiency Index」は、ある特定の試験を受けた大人の英語力に基づいて、国をランク付けします
日本はこのリストで37番目でした。日本は他国と比べて英語力が低い（Japan has a low English proficiency compared to other countries）ということになります。
Of all Asian countries, Japan's English skills are one of the most sadly lacking
This is a statement that presumably, is based on facts. In casual conversation, it may not be necessary to mention the source of this information, however, in a more formal or business setting, you would be required to identify from where this statistic originated.
'........ according to government statistics released in August 2017.'
これは、推定されるに、事実に基づいた説明でしょう。カジュアルな会話では、この情報の源を述べる必要はないかもしれませんが、より公式、ビジネスの場では、この統計の出所を特定することが求められます。
........ according to government statistics released in August 2017.
（2017年8月にリリースされた政府統計によれば、･･･。）
Japan has one of the lowest English skills in Asia
The English proficiency of Japanese people is arguably the lowest in Asia
When you want to explain that the English skills of Japanese people are among the lowest in Asia; then you may express this in the following ways:
-Japan has one of the lowest English skills in Asia
-The English proficiency of Japanese people is arguably the lowest in Asia
日本人の英語力はアジア最低レベルだと説明したいなら、次のように言えます。
-Japan has one of the lowest English skills in Asia（日本の英語力はアジア最低レベルです）
-The English proficiency of Japanese people is arguably the lowest in Asia（日本人の英語力はおそらくアジア最低です）
Of all the countries in Asia, Japan has one of the lowest English skills.
Japanese people seem to have the worst English I've come across in Asia.
Of all the countries in Asia - when saying "of all..." you are comparing one part of a group, to the rest of the group e.g. Of all the students in my class, Alex is the smartest.
I've come across - means in your experience.
Of all the countries in Asia（アジアの国の中で）
"of all..." と言って、グループの一部分と残りの部分を比較しています。
例えば：
Of all the students in my class, Alex is the smartest.（クラスの中で、アレックスが1番頭がいい）
"I've come across" は「私の経験では」という意味です。
Japanese people have one of the lowest levels of English proficiency in Asia.
Out of all the countries in Asia, Japan has one of the lowest rates of English speakers. More people in other Asian countries have a higher level of English. Particularly, Japanese people who speak English are not good at conversational English. This could be because formal study of English in the classroom does not allow for sufficient conversation and oral practice. Instead the focus is on written exercises, reading, and grammar which leaves students able to read and write but lacking in confidence for pronunciation and speaking.
Out of all the countries in Asia, Japan has one of the lowest rates of English speakers. More people in other Asian countries have a higher level of English. Particularly, Japanese people who speak English are not good at conversational English. This could be because formal study of English in the classroom does not allow for sufficient conversation and oral practice. Instead the focus is on written exercises, reading, and grammar which leaves students able to read and write but lacking in confidence for pronunciation and speaking.
↓
「日本は英語が話せる人の割合がアジアで最も低い国の一つです。日本人はアジアの他の国に比べて英語力が低いです。特に、日本人は英会話が苦手です。これは、学校での英語の授業に会話の練習が十分に含まれていないからかもしれません。代わりに、日本では筆記問題や読解、文法が重視されます。その結果、読み書きはできるけど発音やスピーキングが苦手という人が多くなります」
カナダ 
日本 
オーストラリア 
イギリス 
南アフリカ 
アメリカ合衆国 
