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 multilngual, 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.
Links to 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