In Wilfred Owen's poem he ends with "Dulce et decorum est/Pro patria mori". I wonder if there is a recognised term to refer to this technique. Another example of its use is in T.S Eliot's "The Wasteland", where he has lines in German and French such as "And drank coffee, and talked for an hour. Bin gar keine Russin..."
Code switching is used to refer to this, even in poetry:
In linguistics, code-switching or language alternation occurs when a speaker alternates between two or more languages, or language varieties, in the context of a single conversation or situation. — Wikipedia
Wilfred Owen's use of "Dulce et decorum est" is also an allusion to the Odes of Horace.
In The Waste Land the author T S Eliot alludes to Baudelaire's Fleurs du Mal with the line "hypocrite lecteur!—mon semblable,—mon frère!"
Frequently an allusion will be in the form of a direct quotation from another text.
Allusions are often in a language other than the language of the work in which they are found. Yet allusions are even-more-often written in the same language as the text in which the allusion is used.