Isn't this basically what PRECIS (RFC 7564) is about? There are open source implementations of that, like golang.org/x/text/secure/precis for Go (including the predefined profiles for e.g. usernames) or Unicode::Precis for Perl.
In the search engine context, the problem to be solved is that both French and English speakers are likely to type [cafe] and not [café] -- the French speaker because they might be on an English keyboard, or because they know it's not ambiguous.
In the search space, therefore, when you index the word 'café', you also index 'cafe' with a smaller weight. And when you see the query [café], you expand the query to ('café' OR 'cafe'-with-smaller-weight)
And you don't want to do either of this if the two words are actually different!
