The linguistic changes which begin at the Scoto-Anglo border leave descriptive linguists in no doubt that here they are dealing with something other than normal dialect variation.
The Scoto-Anglo political border is also a significant linguistic frontier. It can only be called a language border.

By linguistic criteria there is no doubt that traditional Scots (Braid Scots) is a different language from English, although the distinction between dialect and language owes more to culture and politics than it does to linguistic factors, according to a famous saying “a language is a dialect with an army and a navy”.

There are many examples of mutually intelligible speech varieties being regarded as different languages, and there are several examples of the opposite - speech varieties which are not remotely mutually intelligible being considered as dialects of a single language. In answering the question “language or dialect?”, politics and culture trump linguistics every time.

So while most, if not pretty much all, speakers of the Scots leid* can understand English, the vast majority of English speakers cannot understand Scots in its full focussed braid form.

Obviously all languages gradually change and naturally evolve overtime, even American English has in the past few centuries began developing its own distinct traits from British English, despite much closer comprehension still existing between the two by comparison to that between Braid Scots and 'RP English'.

RP English by the way descends from 'BBC English' - a largely Scottish directed creation on account of Lord Reith of Scotland, whom set the BBC up following his time in the war as an officer of the Cameronians (5th Territorial Battalion of the Scottish Rifles). It basically allowed and enabled a local of Elgin in Scotland to better understand a native of Wrexham in England, who could in turn understand a local of Newcastle in England who could understand a native of Port Talbot in Wales etc.

So essentually, where James VI of Scots authorised the bible's first translation into the English language after he also became James I of England in 1603, thereby accelerating the expansive popularisation of English beyond its shores in the Western world in general, what Lord Reith of Scotland did in short, was create a more 'standardised' universally understood quality of English, which in turn, given the BBCs global reach, also left a lasting impression of this perception of a 'British accent'. Hence many still think of RP English today as an organic default rather than an artificially constructed deliberately directed form of English.

Here he is giving his reasoning in this interview, the one with the facial disfigurement from his time in the war, a result of a shot to the face by a German soldier ~ https://youtu.be/QwBQLoa3E_Y?is=NmrAkiQ6lePzlESM

There are many examples of mutually intelligible speech varieties being regarded as different languages, and there are several examples of the opposite - speech varieties which are not remotely mutually intelligible being considered as dialects of a single language. In answering the question “language or dialect?”, politics and culture trump linguistics every time.

So while most, if not pretty much all, speakers of the Scots leid* can understand English, the vast majority of English speakers cannot understand Scots in its full focussed braid form.

Obviously all languages gradually change and naturally evolve overtime, even American English has in the past few centuries began developing its own distinct traits from British English, despite much closer comprehension still existing between the two by comparison to that between Braid Scots and RP English.

Japanese does the exact same thing as Scots with its use of the Kanji script (Chinese Characters) when simplifying it to form the native Hiragana script, however like Scots to English, the characters are repurposed and have their own seperate definitions in Japanese (Kunyomi) that likewise, could not be understood to a Chinese speaker using a Chinese variety (Onyomi).

Hindi and Urdu are universally regarded as different languages. Urdu is written in an alphabet derived from Arabic via Persian, Hindi is written in the Devanagari script which is indigenous to northern India, so the two look very different on the page. Hindi speakers have no hope of reading Urdu, or vice versa, but this is purely because the two use different alphabets. Despite the different alphabets the spoken languages are perfectly mutually intelligible on a colloquial level, and speakers of Hindi and Urdu can communicate with one another with no difficulty and without any need to learn the other’s tongue.

It’s possible to converse without certainty who's speaking Hindi or Urdu. Issues only arise in the formal form, as Hindi takes formal and literary vocabulary from ancient Sanskrit, most Asian languages derive the way most European ones (save for Basque and the Uralic group) derive from Latin, ancient Greek, and Phoenician prior, whilst Urdu makes more use of words from Persian and Arabic. Hindi and Urdu owe their status as distinct to the fact each has an independent literary tradition - a cultural not linguistic factor.

Each is the official language of a state, these political factors reinforce Hindi and Urdu as distinct. In China the opposite occurs, different dialects of Chinese are different languages from a linguistic view. Cantonese and Mandarin are no more mutually intelligible than English and German but as their speakers share the same written language and common Chinese culture and identity, they're still erroneously regarded as different dialects of one Chinese language.