When it comes to source, you already need to know English somewhat to do any programming (just don’t write your comments and docstrings in Elizabethan pentameter and you’ll be fine).
For graphics, you could put labels etc in a constant Dict{Symbol, String}, and conditionally load language files where this constant is defined. But I don’t know if that’s worth the overhead, if any international users have access to the source they can just replace the strings in place instead. Or you could go the mathematical route and use symbols instead of text: x / m is universal, length in meters is not.