When creating a character and selecting a Background, the list of avilable languages doesn't match the list of available languages that are available to be added post character creation, Netherese for example, amongst others.  Can this be fixed so the same list of languages is available?

I have not yet come across an official 5E sourcebook that offers Netherese as a language. Can you confirm which official sourcebook this is from?

Can you also confirm which Background so we can test ourselves?

Adventurers League Shade Fantatic Background allows it, but my point is that the list of languages for backgrounds doesn't match the list of languages that can be added on the character sheet screen. And it's not possible to add any of these missing languages to a homebrew background either.

Never heard of the Shade Fanatic, which book would that be in? Additionally, you can add languages right in the character sheet itself.

Quote from JohnSolo >>

Adventurers League Shade Fantatic Background allows it, but my point is that the list of languages for backgrounds doesn't match the list of languages that can be added on the character sheet screen. And it's not possible to add any of these missing languages to a homebrew background either.

 
There is no officially released background called Shade Fantatic (Fanatic). There is no official book that presents Netherese as an official language option you can take.

It is therefore not a bug that the sheet doesn't allow selecting it. The sheet is aligned with official rules.

If your DM is allowing you to take Netherese as a houserule then instead of choosing a language option at the normal place (race/background) leave it empty and go to the sheet. Click languages and choose to customise - you can type in any language to add it, be it official or not or made-up.

Quote from GPyromania >>

I imagine you're talking about the Shade Fanatic option here: https://dndadventurersleague.org/bonds-and-backgrounds-for-hillsfar/

That being said, D&D Beyond doesn't currently support Adventurer's League exclusive content. Nothing would prevent you from following Cyb3rM1nd's advice and adding it to your character sheet.

 
Important to note, that site is a support site for AL organisers. While organisers can create AL permissable content which can be used in their AL games, that content is technically homebrew - written for their specific events - and is not WotC official material.

Just for future reference and to help clarify why it's not on D&D Beyond,  in case you use other content from that site.

Then could we not at least have the ability to add languages available on the character sheet to a Homebrew Background?

Blink Dog, Winter Wolf, and other creatures show up as languages when trying to build a Homebrew race, and yet Netherese still does not, nor does Loross, mentioned in Rime of the Frostmaiden.

I'm having an issue with this too. I'm creating a background and I want Thieves' Cant, but it's not on the list. Neither is Auran and a couple others. Help?

Quote from TheGoodKingGeoffrey >>

I'm having an issue with this too. I'm creating a background and I want Thieves' Cant, but it's not on the list. Neither is Auran and a couple others. Help?

 
Thieves Cant is not a proper language. It's a code-system used by thieves in thieves guilds. The tools only permit official languages and Thieves' Cant is not one and the only official way to get Thieves Cant is to take the Rogue class.

Likewise Auran is not a language that a PC can take officially, since it's not really a unique language - it's a dialect of a language. There is "Primordial" language - which is split into the four dialects including Auran. So just give Primordial. 

Interesting. Thanks!

Shouldn't we be able to buy language packs?