Depending on the first language the students have acquired in their life or the language used in class, it is interesting to understand how the language is formed and look at the meanings of words and their origins (semantics and etymology). The question may have to be norrowed down, though. Promoting bilingualism targeting a specific language, or promoting it by means of arousing an interest for learning a (or more) language(s)?

For, to the latter, the analysis, if you will, of semantics and etymology can lead one to pick a (or more) language(s) of one's choice. Moreover, this additionnaly allows for the acquisition of notions that can help to - partly - identify and comprehend the context of encountered languages unknown to the individual.

Besides that, in a more ludicrous fashion comparing semantic networks/lexicon from the initial language to another can invite greater reflexion, and hopefully ignite interest in that other language just as in one's own language.

And just yet both methods can be enlightening and paralleled with the promotion of inclusiveness/opening up to what languages share or oppose in, meaning thus taking into account socio-cultural aspects and so forth that are expressed in them, as if encrypted.

Above all perhaps, the present day language spoken in class is undeniably connect with other languages thus including the language's history, evoking bilingualism. It has happened many times before to have languages meet and undergo linguistic processes and it continues happening today, and so will too in the future.

This again supports the idea of promoting biligualism by comparative linguistics analysis (e.g., focussing on semantics, lexicology, ...).

There needs to be a rationale for doing this and the rationale and potential student outcomes need to be explained to the students and parents. You then teach the language through a subject for example science or mathematics.