Kartotherian's babel does exactly this, but it needs to be configured with the right fallback rules.

@Yurik Could you advise on how exactly to configure Babel's json2tags:// and babel:// in the production sources config file? Thanks!

@SBisson I wrote some docs in https://github.com/kartotherian/babel#usage-examples -- I think prod source configs should follow that pattern. The tilerator config should use the json2tags as part of the tile generation pipeline - converting one giant json string into proper pbf tags. The babel should be used in kartotherian config, to convert the tile on the fly for the user. I would recommend using a language map option with it - to create a proper language fallback map.

@Yurik that makes sense. I guess I was hoping to take a shortcut and make it work with kartotherian only as a first step. This is how I am trying to hook things up:

# Generates vtiles from database
genraw:
  public: true
  formats: [pbf]
  uri: bridge://
  xml:
    npm: ["@kartotherian/osm-bright-source", "data.xml"]

# expand name_ attributes into individual tags
gen:
  uri: json2tags://
  params:
    source: {ref: genraw}
    tag: name

# set name tag based on name_* attributes
genviewraw:
  uri: babel://
  params:
    source: {ref: gen}
    tag: name
    languageMap:
      fr: ['es', 'en']

# View tiles as generated directly from the database. Don't view zoom0-5
genview:
  public: true
  formats: [png,json,headers,svg,jpeg]
  scales: [1.3, 1.5, 2, 2.6, 3]
  static: true
  maxheight: 2048
  maxwidth: 2048
  uri: vector://
  xml:
    loader: "@kartotherian/osm-bright-style"
  xmlSetParams:
    source: {ref: genviewraw}

Does make sense conceptually? Do you see anything obviously wrong?

I get a tile back but it never contains any label. I'm not sure where to look to start troubleshooting...

That seems correct. Try looking at the tile at every state, see if labels are there initially, and if so, when does it disappears