Assuming the second part is picked up by the OSMF it will be Shortbread initially followed up Street Spirit. The former guarantees a stable schema that can be used by others while the latter will include only what is needed by its cartography.
There’s a few sources. A single attribute present on all features in a tile can increase size by 20% and will be worse in most real-world situations. Merging features is also impacted by additional attributes. For example a road split into two ways where the speed limit changes can be merged back into one linestring if you don’t care about the speed limits. If you start putting them into tiles you’re back to needing two linestrings. Merging features can cut the size of tiles in half.
A high-ordinality feature present on buildings would be the worst case. For example, ref:bag is present on many buildings in the Netherlands. Adding it to each building polygon would stop collection into multipolygons and then add an additional attribute on to one of the most numerous features.
This also shows how the total size added is not always a useful measure. If you had a similar tag in Luxembourg it would add almost nothing to the average tile size, but the experience for users in Luxembourg would be much worse.
This is more of a Shortbread question than an OSMF vector tiles one. Shortbread is considering how to support multiple languages best. I’m leaning towards name_XX for all supported languages. This lets styles chose how to manage fallbacks and do things like “Aachen (Aix-la-Chapelle)”. We also may leave picking what languages to support up to people generating tiles. Any style will have to fallback to the plain name attribute, so compatibility is still maintained.
Figuring out what languages to support is tricky. There’s no programmatic way to know what languages are on a feature so a manually curated list is required. With Tilekiln’s templating this wouldn’t be too hard to do.