With recent bud the theme.json can also be generated by bud (which is the case by default).
For having an up-to-date theme.json scanned by wp i18n, the build npm script must run before the translate:pot (or invoked by it) command. Or a separate script that makes bud regenerate only the theme.json could be added that can be invoked before translate:pot (or by it).

@retlehs: Latest stable WP CLI appears to work fine with the used commands now, so this can be merged.

To make this more robust, bud should re-generate the theme.json before the translate:pot script.
This can be achieved by just calling the build script, but wouldn't be just re-generating the theme.json be better than building the whole project? Then bud needs a dedicated command that just re-generates the theme.json (when theme.json generation is enabled in bud, otherwise it can be a no-op).

@kellymears: A dedicated command for bud that only/specifically re-generates the theme.json (or no-op if no theme.json generation is enabled) would be great for this! That command would run before the translation scan, for the translation source strings in the theme.json being up-to-date.

@retlehs, @kellymears: Current stable wp cli 2.8.1 contains a recent enough wp i18n, so this PR can be merged now!