Does DemandTools, PeopleImport and DupeBlocker have support for non-English character sets (i.e. international)?
Yes, all three tools can handle non-English character sets (i.e. double byte and cyrillic).
Be careful of the mapping types used when looking for duplicates. When dealing with double byte characters, avoid the fuzzy and alpha-clean check boxes. Fuzzy is an English fuzzy which will ignore all double byte characters. Alpha-Clean just looks at letters A-Z, numbers 0-9, space and &, ignoring all double byte characters.
Also, avoid special mapping types for Street Address and phone matches (e.g. Relaxed Address Match, Relaxed NA Phone Match, etc.).
For the most part just stick with exact. Cleaned Account Name for Company or Account Names can be used, but it won’t do much unless the suffix, prefix and replace lists have been updated using double byte characters.