It's only the letter H and R (as transcribed in romaji) that are not immediate for a French speaker but they are not difficult. Certainly much simpler than 'th' in English.
(Source: not my ears but Japanese textbooks)
How would a French speaker pronounce し [ɕi] and じ [ʑi]? My guess would be « chi » [ʃi] and « ji » [ʒi]. Close enough to be understood, but different enough to be noticeable.
