Is it possible to write a bilingual poem? I don't mean a poem with an accompanying translation by the poet, but one that is written in one language but contains elements of another language embedded in it. Nor am I even thinking of a situation where a writer inhabits a fully bilingual culture and can depend upon both languages being understood by readers of the poem, as, for instance, a poem written mainly in Welsh but containing words or phrases in English. I want to look at an example of the reverse of that situation: a poem written in English but containing words and phrases in Welsh and published for a readership that includes many with little or no knowledge of the Welsh language. To write such a poem is both a creative endeavour, exploring the limits of bilingual expression, and a statement of cultural affiliation, placing the poem within a tradition that has its main expression in the Welsh language. A poem which sets out to do this is David Jones' The Sleeping Lord. Much of his writing incorporates elements of Latin as well as Welsh into the main language of English. But parts of The Sleeping Lord contain a density of Welsh words that begins to convey the impression of Welsh as the poem's paralanguage and main tonal register.
One of David Jones earliest interpreters, his friend René Hague, expressed reservations about this aspect of his poetry: "His ingenious use of Welsh words as though they had some magical quality reminds one of the Saxon invader in The Anathemata who will "Latin runes tellan....."  Another interpreter, Roland Mathias, offers a lengthy rebuttal of this claim in general terms with better understanding of the Welsh context, but does not offer a specific analysis of the linguistic issues. It is not that the structure of the language used is Welsh. The words used are almost all nouns or short noun phrases. They are often placed in the text so as to take the primary stress both in passages of verse and in passages of prose. So, in The Sleeping Lord,
It is the great ysgithrau of the Porcus Troit that have stove in the wattled walls of the white dwellings, it is he who has stamped out the seed of fire, shattered the pentan-stone within the dwellings; strewn the green leaf-bright limbs with the broken white limbs of the folk of the dwellings, so that the life-sap of the flowers of the forest mingles the dark life-sap of the fair bodies of the men who stood in the trackway of the long tusked great hog, y twrch dirfawr ysgithrog hir. 
In terms of the number of words the use of Welsh here is not great. But Welsh words carry the English sentence's simple base structure and foregrounded subject: 'It is the the great ysgithrau of the twrch dirfawr that has done this'. The foregrounding of important imagery is a characteristic strategy of David Jones' visual art as well as his writing. marking points of symbolic detail in a tangle of variety that is the whole. Here the figure of the legendary hog stands out from the text with the vividness of similar foregrounded figures in his paintings. Seen in this way the use of the Welsh words is as much a matter of technique as it is of cultural affiliation, or rather the two are wedded in a way that René Hague's comment indicates that he was oblivious to.
Elsewhere in the poem Welsh occurs with Latin in long lists or, as below, in a series of reference points in the natural landscape:
and the resistant limbs
of the tough gnarled derwen even
lean to all the briary-tangle
that shelters low
in the deeps of the valley-wood
the fragile blodyn-y-gwynt.
And the wind gusts do not slacken
but buffet stronger and more chill
as dusk deepens
over the high gwaundir
and below in the glynnoedd
where the nentydd run
to conflow with the afon
where too is the running of the deer
whose desire is towards these water brooks.
It is quite true that words are used here, as Hague puts it "as though they had some magical quality". It is also, perhaps, possible to understand why some might not appreciate their function of making the fine detail of the natural landscape specific to a particular place, though it seems to me that, however artful their placing here, they vividly capture that natural landscape in a way that English words would not, and so form the natural ground of the artifice of the poem. Those responding positively to the full resonances of these very specific landscape indicators will not object to Hague's ascription of magical qualities to them. But his "as though" seems strangely wide of the mark. To the reader without some knowledge of Welsh they may appear 'runic', and some have appreciated them at that level. But this characteristic is removed, even for the English monoglot, with careful reading and an easily acquired familiarity with the meanings of the words. Such a reader will be able to follow the rhythms of the lines, taking in the richness of their specifity. It is the skill with which they are woven-in that will appeal, as much as that they stand out. David jones worked assiduously at the careful placing of significant words and would have taken particular care with these Welsh words to ensure their effect and that they were correctly used. The words themselves came to him not because he was in any way a fluent speaker of Welsh, but as a result of his extensive reading and his attempts to learn the language. He told the poet Vernon Watkins: "When I write these names in the stuff I've tried to make they come naturally because by accident I've for long been interested in what they signify in English translations." 
If they also have an incantatory function this is consonant with the author's view of works of art as things "made over" to the gods, giving them a prayer-like function. It is also meant to evoke a complex of associations both with the landscape and with ancestral cultures of the Island of Britain. But he also had a sense of the inherited presence of these cultural references ebbing away. He never expected all of his readers to understand Welsh, but he did hope that the references to Y Twrch Trwyth, Rhiannon, Ceridwen and other names might mean something in the way that references to Classical or Biblical mythology are more commonly used in order to convey the range of associations that go with the act of their naming. In one sense this seems to be a different issue from the use of Welsh words or phrases other than proper nouns. But the act of naming is an attempt at resonance. Using the Welsh word for oak tree (derwen), the fragile wood anenome (blodyn-y-gwynt), meadowland (gwaundir) and streams (nentydd) running into the river (afon) names all the markers of landscape in Welsh giving way only in the final line to 'water brooks'. The English narrative fills in the details of the Welsh scene, the still remaining features of Ynys Prydain inhabited now by speakers of English but the landscape itself singing 'yma o hyd' (still here).
So Welsh words form an outline that is inevitably filled-in in English. They represent an attempt by a writer with little Welsh but that he was able to acquire by extensive reading to firmly identify with the language and its associated culture as well as to affirm the continued presence of the historical roots of that culture in the landscape. One finds no such attempt by many of his near contemporaries among the English-language writers of Wales, although many of them came from Welsh-speaking backgrounds. A fair sprinkling of Welsh place names may appear, or an occasional short quotation in Welsh at the head of an English text, but nothing like the structural reinforcement of English sentences with Welsh substantives that we find in the work of David Jones.
This post is adapted from a piece I wrote in the journal
Materion Dwyieithog/Bilingual Matters 2, 1990 pp.7-9
 René Hague (ed) Dai Greatcoat (Faber, 1980) p.24
 Roland Mathias A Ride Through the Wood (Seren, 1985)
 David Jones The Sleeping Lord (Faber, 1974)
 David Jones (ed. Ruth Pryor) Letters to Vernon Watkins (University of Wales Press, 1976)