This should be quite easy with pywikibot if the file-names and translations to add are given. 
If there are no indonesian translations currently in the descriptions the new translation can simply be appended after "|information="

Mvg, 

Bas

> From: faewik@gmail.com
> Date: Mon, 9 May 2016 12:36:19 +0100
> To: j.m.muller@hccnet.nl; glamtools@lists.wikimedia.org
> Subject: Re: [Glamtools] adding 3134 captions after upload?
>
> There's no 'painless' solution. I have made similar housekeeping
> updates to very large batch uploads post-upload using pywikibot using
> short scripts. Amongst other things, pywikibot can pull up the
> specific image page history and tell which past edits have been made
> and by whom, then make intelligent decisions on whether to add
> information or how to best do it. Terribly handy for avoiding doing
> something twice.
>
> I suggest not being in a hurry. Focus on getting the translations
> done, if amenable to it, it would be worth training up the translator
> so they write the new text directly into the image templates rather
> than somewhere else. If this would be too difficult, then just get the
> translations as a file or spreadsheet, mapped against the related
> Commons images or reference numbers. There are several bot writers
> that might be interested in plugging the extra translations seamlessly
> into the current image pages once the data is ready. The best place to
> ask for a Commons bot specialist is on the bots work requests page.[1]
>
> As you do so much of this stuff, I strongly recommend you try playing
> around with Python and pywikibot yourself, perhaps delaying this
> translation fix so you can use it to practice with. A script to do
> what you are looking for might only take around 20 or 30 lines of
> original programming, most of pywikibot's power lies in the handy
> modules that have been written to do exactly this sort of thing. See
> the advice and code examples in the manual.[2]
>
> Links
> 1. https://commons.wikimedia.org/wiki/Commons:Bots/Work_requests
> 2. https://www.mediawiki.org/wiki/Manual:Pywikibot
>
> Fae
>
>
> On 9 May 2016 at 12:19, Hans Muller <j.m.muller@hccnet.nl> wrote:
> > Dear All,
> >
> > ** Is it possible to add 3134 captions to existing images on Commons?
> > ** (without removing earlier work on the images, metadata and categories)
> >
> > This nice question presents itself in the following case. Recently, i
> > uploaded 3134 images from Dutch University Library Leiden UBL and KITLV to
> >
> > https://commons.wikimedia.org/wiki/Category:Media_contributed_by_KITLV_2016_-_Art_-_Temp
> > (309 colonial works of art)
> >
> > and
> >
> > https://commons.wikimedia.org/wiki/Category:Media_contributed_by_KITLV_2016_-_Temp
> > (2805 colonial photographs)
> >
> > with captions in Dutch and English. I corrected the metadata, categorised
> > the images and put them into Wikipedia articles (only a few ;-)
> >
> > Now a translator might be paid to translate the captions into Bahasa
> > Indonesia, the language of the people perhaps most interested in these
> > historical images. If the translator succeeds, that would mean require
> > language templates like
> >
> > {{id|1= ... }}
> >
> > to the description fields.
> >
> > Re-uploading with GWToolset proved to be impossible if another user has
> > worked on the metadata since upload. Categorisation and my own corrections
> > to the metadata would be lost.
> >
> > * Is there another, painless solution putting in the new translation while
> > keeping the previous work on these files?
> >
> > Thanks for you considering this problem,
> > Best regards from a sunny Utrecht, the Netherlands,
> >
> > hans muller
> > https://commons.wikimedia.org/wiki/User:Hansmuller
>
> --
> faewik@gmail.com https://commons.wikimedia.org/wiki/User:Fae
>
> _______________________________________________
> Glamtools mailing list
> Glamtools@lists.wikimedia.org
> https://lists.wikimedia.org/mailman/listinfo/glamtools