The Flemish settlement in Pembrokeshire was a movement of Flemish settlers in England to Little England beyond Wales, mostly in south Pembrokeshire in southwest Wales. This was a systematic plantation by Henry I and Henry II, as a way to displace troublesome Flemish refugees and to influence Welsh affairs. Although it is believed that they created space for the English language to predominate over Welsh in the area, there is no known influence on language in the area.
William of Normandy.
Although Saxons were among the first foreign settlers there, steering the language, quite a few Flemings also arrived after the Norman conquest of England. William of Normandy had been married to Matilda of Flanders and quite a number of Flemish nobles and soldiers had joined William in 1066.
First wave: 1108.
In Henry I's reign in the early 12th century a wave of Flemish refugees arrived in England, escaping damage from floods that affected the coast of Flanders. At first they were given land on the eastern side of the Tweed before Henry I systematically planted the Flemish settlers in south Pembrokeshire. 
Early documentary evidence of Flemish immigration from England, rather than directly from Flanders, is given by contemporary William of Malmesbury (1095–1143), who wrote: The fact that they came via England, and that at that time the Flemish language was not markedly different from Old English, was likely to have influenced the English language becoming, and remaining, the dominant language of the area.
A contemporary writer, Caradoc of Llancarfan (fl. 1135), was more explicit:
Second wave: 1113.
A second batch of Flemings were sent to southwest Wales by King Henry I in about 1113. According to "Brut y Tywysogion" he:
Henry II.
Henry II (reigned 1154-1189) equally sent Flemish people, mainly soldiers and mercenaries, to Pembrokeshire, but this was mainly driven by the desire to have the warmongering Flemings (the "Flemish wolves") out of his way. As an increasing number of "foreigners" settled, the original inhabitants were driven away. It has been called a "process of ethnical cleansing". The Flemings showed a real zest for settling elsewhere, discarding the social fabric that was in place: they were "a brave and robust people, but very hostile to the Welsh and in a perpetual state of conflict with them". 
In 1155, under the orders of the new King Henry II, a third wave of Flemings were sent to Rhys ap Gruffydd's West Wales territories.
High Middle Ages.
What followed, starting with the reign of Edward I in the late 13th century, was 100 years of peace, particularly in "Little England", marked by the Edwardian conquest of Wales, which must have compounded the tendency of Welsh to become a minor language in the region.
With the failure of Owain Glyndŵr's revolt in the early 15th century, in which no fighting took place in "little England", came punitive laws affecting Wales, though these were, for reasons historians have not been able to ascertain, applied less rigorously here than elsewhere in Wales.
Local influence.
Haverfordwest and Tenby consequently grew as important settlements for the Flemish settlers. In Tenby, a castle and a church was erected for the Flemish colonists.
The Flemish were experts in the woollen trade, and this flourished in the area.
The Normans and the Flemings built a line of over 50 castles – most of them earthworks – to protect south Pembrokeshire. This line of castles is known as the Landsker (old Norse for 'divide') and stretched from Newgale on the west coast to Amroth on the south east coast. The Landsker line represented a divide in language and custom in Pembrokeshire that remains tangible today.
Rise of English.
The Flemish language did not survive in the local dialect. Ranulf Higdon in his "Polychronicon" (1327) stated that Flemish was by his time extinct in southwest Wales, and George Owen in 1603 was adamant that Flemish was long extinct.
Although the initial planting of Flemish was a move by Norman rulers, the influx of Flemings into south Pembrokeshire appeared to be so significant that Flemish allowed for English to become the dominant language in the region.