I saw that the thread grew cold (for some reasons, interesting discussions start and end during the few periods where I have no time to check the forum) and I have no time to read the answers offered by others, so I will be brief and focus on only two points.
1) a work based on secondary sources is certainly derivative, but it is not necessarily useless. For instance, you can provide new insights on points neglected by the authors who dealt with them previously, or reach new conclusions, or correct factual mistakes, or simply provide an essay which lists all the available bibliography. The really important thing is to be honest about the nature of the work.
2) as for the languages, in principle one should not work on subjects which require the knowledge of languages that one does not master: one cannot do a solid scholarly work on the Russian army without knowing Russian. On the other hand, one can bypass the problem by working on a specific part of the subject: if you are interested in the Swiss, why don't you work on Swiss troops in - say - British service ? On the other hand, seldom a scholar masters ALL the languages his field would require (I do not know how many Napoleonic scholars can read French, English, German, Russian, Italian, Spanish and perhaps a few of the smaller European languages). I think it is perfectly fine to know and use a few languages, and then rely on translations or secondary literature as far as the others are concerned. Again, being honest about that is the main thing.

Best wishes,

Giorgio