When it comes to English, if you don't perfect the basics, you won't be able to use it in practical situations
As for English, it is important to perfect the basics, otherwise you won't be able to actually use it in real life
「英語において」＝ When it comes to English... / As for English... / Regarding English...
「基礎」＝ the basics / the fundamentals / the foundation
「完璧にする」＝ perfect
「応用が利く」＝ apply your knowledge / put it to use / put it to practical use / use it in real situations
「英語において基礎を完璧にしないと応用が利かない」＝ When it comes to English, if you don't perfect the basics, you won't be able to use it in practical situations
As for English, it is important to perfect the basics, otherwise you won't be able to actually use it in real life
イギリス 
