Let's suppose you want "to properly learn history of" England, and that you do speak and read English.
How far back will you want to research?
A thousand years? Then you'd certainly read Beowolf in its original English:
Hwæt! wē Gār-Dena in geār-dagum,
þeod-cyninga, þrym gefrunon,
hu ða æþelingas ellen fremedon.
Oft Scyld Scefing sceaþena þreatum,
monegum mægþum, meodosetla ofteah,
Well, perhaps you're not that interested in history that goes back that far after all.
How about only 400 years ago, when the Early Modern English language was at its prime with Shakespeare and the King James version of the Bible.
Such as Hamlet's soliloquy:
To be, or not to be, that is the Question:
Whether 'tis Nobler in the minde to suffer
The Slings and Arrowes of outragious Fortune,
Or to take Armes against a Sea of troubles,
And by opposing end them: to dye, to sleepe
No more; and by a sleepe, to say we end
The Heart-ake, and the thousand Naturall shockes
That Flesh is heyre too? 'Tis a consummation
Deuoutly to be wish'd. To dye to sleepe,
To sleepe, perchance to Dreame; I, there's the rub,
For in that sleepe of death, what dreames may come,
When we haue shufflel'd off this mortall coile,
Must giue vs pawse. There's the respect
That makes Calamity of so long life:
For who would beare the Whips and Scornes of time,
The Oppressors wrong, the poore mans Contumely,
The pangs of dispriz'd Loue, the Lawes delay,
The insolence of Office, and the Spurnes
That patient merit of the vnworthy takes,
When he himselfe might his Quietus make
With a bare Bodkin? Who would these Fardles beare
To grunt and sweat vnder a weary life,
But that the dread of something after death,
The vndiscouered Countrey, from whose Borne
No Traueller returnes, Puzels the will,
And makes vs rather beare those illes we haue,
Then flye to others that we know not of.
Thus Conscience does make Cowards of vs all,
And thus the Natiue hew of Resolution
Is sicklied o're, with the pale cast of Thought,
And enterprizes of great pith and moment,
With this regard their Currants turne away,
And loose the name of Action. Soft you now,
The faire Ophelia? Nimph, in thy Orizons
Be all my sinnes remembred.
— Hamlet (Folio 1, 1623) :: Internet Shakespeare Editions
Or Paul's description of charity:
And though I bestowe all my goods to feede the poore, and though I giue my body to bee burned, and haue not charitie, it profiteth me nothing.
Charitie suffereth long, and is kinde: charitie enuieth not: charitie vaunteth not it selfe, is not puffed vp,
Doeth not behaue it selfe vnseemly, seeketh not her owne, is not easily prouoked, thinketh no euill,
Reioyceth not in iniquitie, but reioyceth in the trueth:
Beareth all things, beleeueth all things, hopeth all things, endureth all things.
Charitie neuer faileth: but whether there be prophesies, they shall faile; whether there bee tongues, they shall cease; whether there bee knowledge, it shall vanish away.
— 1 Corinthians Chapter 13 KJV
The Bible was deliberately translated with simple words to make it more accessible, so it's much more understandable than the Shakespeare.
But, some of those words have different meanings now than what they had when this was written.
The first line for instance no longer makes sense: If I give everything I own to feed the poor … but don't have charity ….
Doesn't "charity" mean giving to feed the poor?
Yes, it does now, but it didn't when this was written.
If you read a word whose meaning has changed, and don't realize that it has changed, you'll totally misinterpret the text.
Many words in the KJV no longer mean what they did when they were written.
And think of the words that are retained in songs that we still sing today: "don we now our gay apparel" means something quite different from what it meant only a century ago.
And what does "Naught be all else to me, save that thou art" mean?
All those words are used with their current meaning, but the structure of the sentence is like nothing one would say today.
(See what to do when words change in meaning.)
The point is, that being fluent in a language is only of limited help in studying history.
If you want to make use of it to study original documents, you're going to have to learn all the ancient version of it too.
It's much easier and, unless you study so much that you become a famous scholar in ancient languages, more reliable to simply trust the work that others have done to translate the original works into the modern version of the language.
But if you're going to do that, why not read works that have been translated into modern English, and not bother learning the modern version of a language that isn't going to help understand the original works anyway?