In correct English usage "I shall" and "We shall" are, respectively, the first person singular and plural of the future tense of the verb "to be". The use of "will" instead of "shall" in these cases either makes the verb into the subjunctive or is purely an example of lazy speech/writing.
Being picky if it is just the future tense it should be "we shall return" . " We will return" carries a meaning of being definite, or determined about it. I'm afraid it's one of the ways that sloppy English diminishes the language, although making it more approachable to those learning it,
In Brazilian Portuguese, «l»s will commonly be pronounced as [w] in IPA. However, the vowel quality of the two verbs maintain an important distinction: the «o» in «voltar» sounds like "aw" in the American English "law" or [ɔ] in IPA (even though it kinda sounds like [o] here), while the «o» in «votar» sounds like the "oo" in "too" or [u].
'we will be back' is quite proper in grammatical English, but it has a different detailed meaning to 'we shall be back'. 'We shall be back' is the proper use of the future tense. 'We will be back', is a dogmatic statement of intent and as such is very much a definite stament that we shall be back under any circumstances.