OED, rather than listing 'ya' as a form of 'you' and 'your', gives 'ya' (pronoun and adjective) separate entries with a distinct origin and etymology. In both cases, the origin is a "variant or alteration of another lexical item" ('you' and 'your') and the etymology is that they represent "a regional or colloquial (chiefly unstressed) pronunciation" of 'you' and 'your', respectively. Neither the pronoun nor the adjective are reported as chiefly US or UK.
The first OED attestation of the pronoun is from "Exmoor Scolding I" as published in the 2 June 1727 Brice's Weekly Journal (Exeter, Britain). "Exmoor Scolding" contains a representation of 18th century Exmoor dialect in narrative form. In a 1782 republication what the first publication in 1727 spells more commonly as "pritha" has become "pitha" and "rezinable" has become "reaznable".
Pitha tell reaznable, or hold thy Popping; ya gurt Washamouth.
The 1782 republication of "Exmoor Scolding" also provides a glossary.
OED first attests the adjective 'ya' with the glossary entry for 'random' in William Carr's 1824 Horæ Momenta Cravenæ, or, The Craven Dialect:
Random, To be in a straight line or direction, "let ya fence random wi' tother."
From such evidence it is a virtual certainty that the "regional or colloquial" pronunciation of both the pronoun and adjective represented in publications as 'ya' dates to the 1700s, and that neither 'ya' owes much, if anything, to the "influence of a large population of people that still speak German".