Support Japanese (CJK) characters in the screen fonts

What I would like

Japanese characters in the screen fonts, so tile names and entity states written in Japanese show on the glass instead of empty boxes.

Why it matters

Home Assistant users in Japan name almost everything in Japanese: rooms, entities, scripts, scenes. Today every tile name has to be rewritten in English just for the screen, which breaks the link with the rest of Home Assistant and makes the panel harder to read for family members who do not read English.

The precedent

"Support cyrillic character set" shipped in Tessera 0.4.91 (firmware 0.58.0), with the Cyrillic letters built into the screen fonts. This is the same kind of request, with one extra problem: the number of characters.

The hard part, and a way around it

The fonts list their glyphs one by one, and a screen carries roughly eight sizes (headline, label, sublabel, setpoint, watch value and so on). A full kanji set (2,136 joyo kanji, or 2,965 for JIS level 1) across all of those sizes would be several MB, far more than Cyrillic's 66 letters. I understand why this is not simply "add a font".

But a full set is not needed for a home panel. Three options that would already help a lot:

1. Kana only (hiragana + katakana, about 180 glyphs). Cheap, and already covers many common names.
2. A per-screen glyph list. Tessera builds firmware per screen and already knows every tile name at build time, so it could collect the characters actually used on that screen and build only those. A household panel rarely uses more than a few dozen kanji. This scales to Chinese and Korean too, without a fixed cost for everyone.
3. Larger sets on boards with the memory for it. The M5Stack Tab5 has 16MB flash and 32MB PSRAM, and its current firmware is about 4.5MB. A wider set is realistic there even if a CYD cannot hold it.

Option 2 looks like the best fit for how Tessera already works, and it would cost nothing for users who never type a non-Latin character.

Happy to help

I run Tessera on a Guition 4 inch and an M5Stack Tab5 and can test builds on both. I can also provide a Japanese translation of the UI strings if that becomes useful later.