This timeline explores if the Germanic Languages didn't diverge nearly as much as in OTL. There will probably not be any pictures, but if I can find any I'll add them.
Emergence of German-Celtic languages (100-500 BC)[]
It's generally excepted by Linguistics experts that in the beginning the Celtic and Germanic languages were all similar (or mutually intelligible) and some understanding has even been thought to have been between these language groups and the Slavs, a people who lived to the East. Artifacts dating back to nearly 200 BC show that the Slavic and Germanic peoples seem to have been able to communicate some very basic things via writing on rocks (hello, goodbye, I will give you this for this, etc). Artifacts from around the same time also show that Germanic and Celtic languages were very much understandable to speakers of said language groups.
The fall of Rome and the rise of the Anglo-Saxons (500's-800's)[]
When Western Rome fell in the late 400's Briton was the primary language spoken in OTL England. By the mid 500's however, it is shown that contact with Saxon traders led to much Germanic influence on Briton, leading to the emergence of Anglo-Saxon sometime in the 600's. Anglo-Saxon was considered a bridge between Celtic languages like Scottish and Welsh and Germanic languages like Saxon and Prussian. Over time (mostly in the 800's) Anglo-Saxon had lost much of the historic Briton influence and during this era could be called a mere dialect of Saxon (the higher-ups of England actually used standard Saxon).
The Norse language & dialects of Norse (700's-present)[]
The Norse Language emerged in the late 600's but wasn't used much until the mid 700's. Norse evolved into 4 distinct dialects over time: The Danish-Norwegian Dialect, the Skane dialect, the Stockholm dialect and finally the isolated Gorky dialect (extinct since the early 1600's, by 1300's it was really a separate language, with very little understanding of other Norse dialects). The modern Gorky accent has much influence, however, from this extinct language and sounds completely different from the surrounding Russian accents.
German (parent to all Languages above, considered the standard Language of the Germanic Languages)[]
German is a language spoken from the North of Italy all the way to the eastern most reaches of Prussia and also as far West as OTL Belgium & Alsace Lorraine. The language is considered the standard Germanic language and unifies all the dialects and languages under a common language.
Middle & Modern Anglo-Saxon (800's-present)[]
Anglo-Saxon has evolved over the years in some pretty major ways. It is still intelligible with German and the Norse languages, but it's features can be hard to grasp for speakers of these related languages. Modern Anglo-Saxon uses quite a few of it's own words and phrases and even borrows from French in some minor ways; but, to a speaker of German or Norse it's still mostly understandable. Anglo-Saxon is the largest and most used language of the Germanic languages, but German is considered the Standard.
Normandic (late 1100's-present)[]
Normandic is closely related to Anglo-Saxon and Anglo-Saxon is what Normandic has borrowed quite a lot from. Normandic is debated between being a Romance language or a Germanic one. Normandic doesn't have nearly as much understanding between it and Norse and German as Anglo-Saxon does, but Normandic and Anglo-Saxon are pretty understandable.
North American Anglo-Saxon[]
North American Anglo-Saxon is spoken in Canada and the USA and is not understandable to a German, except very select and oddly random words (like Cheese), but Anglo-Saxons from England can understand most of what North American Anglo-Saxons are saying and Norse and Normandic speakers can get around 40% of what's being said.
The Swiss Language[]
The Swiss Language is very heavily influenced by the Romance languages and is not Mutually Understandable to any other Germanic Language. Very few words are even the same as Swiss has either made up it's own words or just uses French ones. It's assumed that from it's first divergence in around 510 to about the late 1500's-early 1600's that there was (although deteriorating throughout this time) at least some understanding between Swiss and other Germanic languages and that Swiss just took a distinctly different route over the millennia.
Modern Day Intelligibility between Germanic and Celtic languages[]
Over the thousands of years there has, of course, been quite a divergence between the Celtic and Germanic languages. However these groups of languages are still semi-understandable, particularity Welsh and Anglo-Saxon, where speakers can even sometimes get over 70% of what the other is saying, however if a speaker of one group of languages doesn't want a speaker of the other to know what they're saying, it's easy to use unique words and throw off the other speaker.