[quote="Eric R"]
all the Spirits are gender-neutral - or more accurately, gender-NA. "His" should be "its", or optionally "their".
[/quote]
Does this imply that the Dahan language has no distinction in gender in its pronouns (sort of like Chinese, at least verbally) or do they have a gender neutral pronoun (like Russian)?
If the latter, when Thunderspeaker takes human form, do the Dahan refer to them with the pronoun that they manifested as, or as gender neutral?
I believe Eric said somewhere that they have specific forms of address when talking about/to Spirits.
That's cool, in my Quaker heritage the idea of "plain speech" was largely designed to set aside the formal pronouns for use only when speaking of God, so having a culture that developed around god-like entities having special pronouns for them is really cool.
I figured Thunderspeaker appeared to the Dahan as female because it chooses so, and that therefore it wanted to give some impression to them. Maybe the Dahan are somewhat matriarchal?
"Somewhat matriarchal" is a pretty good descriptor for the Dahan. Their families / households are formed around (and headed by) women. Formal clan/village leadership is not gender-associated, though the chief is a semi-proxy for their family, so it's a little muddy there. War-leader(*) is also not gender-associated, but is often drawn from those-who-travel, who statistically lean slightly male due to some women not wanting to take on far-ranging roles during late pregnancy and early motherhood and heads-of-household (who are always women) nearly always being among those-who-stay. Other roles of leadership and/or prestige that I can think of offhand aren't gender-associated and don't skew either direction across the Dahan as a whole.
(*) = The normal Dahan version of "war" is a far cry from the modern usage, and what Thunderspeaker leads the Dahan in is not considered "war"; "raiding" would likely be the closest translation. This is why "Call to Bloodshed" isn't named "Call to War"… and "The Trees and Stones Speak of War" is the Spirits/land advising the Dahan not just on tactically useful information, but on the different way in which the Invaders are approaching this conflict.
Thunderspeaker appears as female or male or neither or who-knows because it chooses to, but that choice may not have deep import. Some days you pick clothes to make a particular impression, but some days you just throw something on because it's comfortable. Given its nature, it's much more likely to be forming its guise based loosely on Dahan it once knew who had exceptional voices than as any sort of social message! 
(Particularly since gender among the Dahan is mostly relevant in matters of family - marriage, households, having kids, kin relations, family status, etc - and Thunderspeaker isn't really hooked into that part of Dahan society.)
(All the above is in-world, and separate from the real-world reason we chose to portray the primary Thunderspeaker art as female - half "why not?", half "counterbalancing existing male skew".)
Man, I absolutely love getting new pieces of lore for this game.
I feel the principle of "you only show a tenth of what you write" is coming into play with this game. You've thought through an awful lot of this stuff we're only just now starting to think about
Yeah, that’s excellent lore-stuff!