I'm writing this thread to first and foremost, explain the origins & the times at which certain proto languages where spoken, & when subbranches split, in order to show an overall better understanding to those trying to understand the bronze age through dark ages (yes I still use the term dark ages as it's a useful term for the period between the fall of rome and the battle of hastings)

-Proto-Germanic
proto-Germanic began to be spoken about 500 bc, and branched off into the 3 main branches by 250 bc. Despite this relatively recent time-frame, the predecessor of Germanic, pre-proto-Germanic, had likely been spoken by the Nordic bronze age culture, and was much closer to classical indo-european languages phonetically, only having undergone minimal changes until the proto-Germanic stage. Proto Germanic itself started out heavily restricted to the Jutland
(you-t-land, not jew-t-land) peninsula, southern Scandinavia, and the islands in between. They began to expand into the Hercynian forest, down the Rhine and Elbe about 400 bc, slowly assimilating and destroying the central european celts as they went.
by 300 ad, Gothic and proto-Norse had become their own branches, and the Frankish, Anglo-Frisian+low German, & high German branches began to develop into more recognizable forms. around 400 ad, the Saxons, Angles, and Jutes began their domination of the land now known as England. Around the same time, proto-Norse from Zealand began to replace the jutish dialects of Anglo-Frisian in Jutland, beginning what would become Danish.
In Gaul, much of the Frankish language was replaced by the native aristocracies spoken Vulgar Latin, becoming the Gallo, or Oil branches of Italic. Frankish however continued to be spoken in the low countries eventually becoming Dutch.

-Proto-Celtic
even though many scholars have said throughout the years that proto-Celtic began in the alps of central Europe around 1200-800 bc, recent evidence has shown that it's more likely that proto-Celtic was spoken by Atlantic traders along the coast of Iberia, armorica(Brittany), & the British isles somewhere between 2000 & 1200 bc.
The language spread inland, assimilating similar Italo-Celtic dialects spoken in Gaul and the alps (ligurian) around 1200 bc, and adopted the Hallstatt & La Tene style artistic culture through trade back to their homeland. These Celtic peoples than spread to the Italian peninsula, and the Balkans, in a last great multi-tribal conquest around 500 bc.
Many of these tribes where assimilated by early Romans and Illyrians not long after. Although a small population of these invaders made it to the central Anatolian highlands founding the kingdom of Galatia. The Galatians where often hired by Greeks, Egyptians, Canaanites & Babylonians as mercenaries, before eventually being Hellenized.
Somewhere between 1200 & 900 bc, the Q-Celtic languages (Gaelic, & Hispano-Celtic) likely split from P-Celtic (Gaulish & Brythonic), the Q-Celtic languages being the more archaic do to isolation after the break up of the Bronze age Atlantic trading culture, and remained very archaic till about 400 ad when Primitive irish began to go through dramatic changes into Old Irish.
Gaulish likely remained a peasant language in northern France until about 500 ad, and it was around this same time that Brythonic began to go through dramatic changes into late proto-brythonic, which is the common ancestor of Welsh, Cornish, Breton, and Cumbric, with Welsh & Cumbric making the northern branch, as well as Cornish & Breton making the southern branch. Cumbric died off sometime between 1000 and 1300 in southern Scotland & Cumbria. its noted that William Wallace was likely a part of one of the last generations of Cumbric speakers (Wallace means 'Welshman' in northern old English or old Scots).
Around 600-700 ad, Gaelic was spread to the isle of Man & western Scotland (Dal Riata). Gaelic soon replaced the Manx dialect of brythonic and Pictish, which was likely just the northern most brythonic dialect. Pictish died out in the east of Scotland around 800 ad.
The ranges of all celtic languages have been heavily declined over the past 2000 years, and is now restricted mostly to Wales, the Gealtacht & the Isle of Lewis.