The new UK government advice on Ks2 Foreign Languages in English schools (2014) states that teaching may be of any modern or ancient foreign language, enabling pupils to make progress in one language.
Although this presents challenges to a school pursuing a mulitilngual, investigative approach, it is not impossible, and may be preferable to maintain such an approach as long as a specific target language for learning has been identified.
As those of us who have used investigating languages materials over the years know, in using the materials, we can make strong links to English and other areas of the curriculum, we can value all languages within our school community, and a teacher, who is not proficient in a foreign language, can deliver the approach based on his or her expertise in English.
UK Government advice:KS2 Foreign Languages (2014)
The National Curriculum for Foreign Languages in Years 3 to 6.
Note that the curriculum aims state that: Teaching may be of any modern or ancient foreign language and should focus on
enabling pupils to make substantial progress in one language
FL2/1.1 Listening & Comprehension
1.1a listen attentively to spoken language and show understanding by joining in and responding
1.1b explore the patterns and sounds of language through songs and rhymes and link the spelling, sound and meaning of words
1.2a engage in conversations; ask and answer questions; express opinions and respond to those of others; seek clarification and help*
1.2b speak in sentences, using familiar vocabulary, phrases and basic language structures
1.2c develop accurate pronunciation and intonation so that others understand when they are reading aloud or using familiar words and phrases*
1.2d present ideas and information orally to a range of audiences*
FL2/1.3 Reading & Comprehension
1.3a read carefully and show understanding of words, phrases and simple writing
1.3b appreciate stories, songs, poems and rhymes in the language
1.3c broaden their vocabulary and develop their ability to understand new words that are introduced into familiar written material, including through using a dictionary
1.4a write phrases from memory, and adapt these to create new sentences, to express ideas clearly
1.4b describe people, places, things and actions orally* and in writing
1.4c understand basic grammar appropriate to the language being studied, including (where relevant): feminine, masculine and neuter forms and the conjugation of high-frequency verbs; key features and patterns of the language; how to apply these, for instance, to build sentences; and how these differ from or are similar to English.