If you really wanted to clutter up the drawings, you could do all the notes in dual languages too. Alternatively, to clean up the drawings, why not show the weld sizes in the primary system or units, and provide a conversion table in the general notes or near the title block. Management always has really good (wild, crazy, necessary, maybe not??) ideas to solve or address one problem, that then generate a hundred new problems. So, make these moves, or insist on these changes only after some considerable study of the consequences.