Hi Peter,
I know both systems very well and I would not use DSpace for journal management, mainly if you have already a OJS instance.
First, you should investigate better the problem you identify regarding the limitations about OJS handling many languages. It is actually very versatile regarding this aspect and don't know what is the problem, you may describe better the limitation.
Then, OJS includes by default a big set of features and plugins to improve the journal integration with other services (DOI, Preservation, DOAJ, ...), and more important, it includes standard workflows to support an editorial activity based on good practices. One example, you can't reproduce on DSpace, is the review process where you first ask if the reviewer is available, if he responds yes, so it can start the review. The review process itself includes some instructions for reviewers, comments for the author and/or editor, can use evaluation forms. Additionally, when you select a reviewer, it must have that role and you know if it has other active reviews, the area of specialty, ...
Finally, it allows the editorial team to access each work and understand all the steps that have been done, all the emails that have been sent, ...
Some journals don't use the workflow of OJS... it's a bad practice, and maybe they need some initial training and support to allow the use of the workflow.
Regarding the indexing requisites, you usually need some pages (which you can't add by default on DSpace) to explicitly show the editorial board, policies, ... Also, some services may request access to check the review process.
In terms of indexing in search portals, they are very similar, and both have OAI-PMH, so I don't see limitations in this context.
OJS had a recent change from versions 2.x to version 3.x which is similar to what is happening now with DSpace 6.x to 7.x in the way that the change technically all the system to use more recent technologies.
This is my opinion and others are welcome to share their experiences.
Regards,
José Carvalho