Use e.g. the Mkgmap option:

--name-tag-list=name:en,int_name,name

This prefers English or International name tags first before falling back to the normal name tag.

Alternatively, you can also download maps from http://garmin.na1400.info/routable.php which are transliterated in addtion to the option above (transliteration is the translation of foreign languages to latin characters while adhering to official conversions).