The relevant section in wild form (PHB 67) states:
… and your ability to speak […] is limited to the capabilities of your beast form.
The relevant section of the Compendium of Wisdom Advice 2019 (page 3):
Can a druid from the circle of the moon speak the languages
know in the form of an elemental?
the elementals listed in Elemental Wild Shape can speak.
A literal interpretation (RAW) of Wild Shape could
reasonably make you think that the druids transformed
only speaks languages that appear in the language of an elemental.
stat block, but the intention (RAI) is that the druids retain their
knowledge, including languages, when they change
and can speak the languages they know if an adopted form
So, you keep your knowledge of all languages (this includes do not knowing the giant momentum).
However, it is unclear if this is a limitation of the capabilities of the form of the beast it prevents him from speaking the three languages or anything else.
My opinion on the question, as a DM
If it is the physical abilities of the creature that prevent it from speaking these 3 languages (we see this in the real world with dolphins, for example, who speak "dolphin", but can also understand humans, but do not do not know how to speak English), so we might assume Elks giants work the same way and are therefore limited to speaking no language.
If it is a mental thing that prevents the creatures from speaking the 3 languages, it does not make sense that a new spirit inhabiting the body of a giant Elk would be subject to the same limitation.
So I would say either you can talk no languages, by slashing the RAW decision, or could talk about all the ones you know, ignoring the possible decision to restrict the language.