LANGUORETH (this is not a modernized form)
As mentioned above in the discussion on Den(y)w, her son S. Kentigern interacted (according to his biographers) with Rhydderch Hael, king of Strathclyde, and his wife Languoreth. Rhydderch is given a calculated date of ca. 540.
Textual Sources
As far as I have been able to discover, the only mention of this woman by name is in Jocelyn's Life of S. Kentigern (written ca. 1185) -- the fragmentary anonymous Life of Kentigern does not include this episode. Jocelyn's work survives in two early manuscripts and the name appears in slightly different forms (Forbes 1874).
Languoreth - British Museum Ms. Cotton Vitellius C. viii.Languueth, Langueth - Archbishop Marsh Ms. V.3.4.16 (Dublin)
An unscientific survey indicates that Languoreth is preferred by most modern scholars.
Linguistic Analysis
The linguistic forms found in Jocelyn's work preserve certain spelling features that suggest a source around the 7-8th century. Assuming the name here is a typical dithematic compound, we may separate it into Lan + guoreth. The first element is not easy to find parallels for in personal names. For the second element, it is strongly tempting to read the final th as an orthographic variant for t as in Thaney/Taneu, and then to connect guoret with the second element of the masculine given name Tegwared, and possibly also to a word meaning "deliverance, redemption, help". The first part of the name is unlikely to be identical to the common noun llan "enclosure, esp. a church" unless the name is somehow an allegorical phrase "the redemption of the church". But the phonetics of the first element don't present any significant ambiguities. In the mid 6th century, we'd expect a Latinized written form along the lines of Lanuoret and a pronunciation along the lines of ['lan-wor-Ed] or, using English syllables, "LAHN-wohr-ed".