The Translate extension contains a couple of references to theme-wikimedia-ui-legacy.less from the @wikimedia/codex-design-tokens package. It looks like this file (or at least some of the tokens that it contains) is being vendored into the Translate extension source code.
Recent versions of Codex have eliminated all differences between theme-wikimedia-ui-legacy.less and the default theme-wikimedia-ui.less file. We are in the process of removing all traces of the "legacy" set of tokens from Codex (see T360806).
For the time being, Codex will continue to produce a copy of the main theme-wikimedia-ui.less design tokens file under the old "legacy" name, for backwards compatibility purposes. However, this file will eventually be removed (probably at the same time we do an official Codex 2.0 release).
The Translate extension front-end code should be updated to remove these references in favor of the standard theme-wikimedia-ui.less file.