Tiny misunderstanding, what Mapparium (and MUIMapparium) does. It renders the tiles online (in the Cloud, SCNR) and only download the ready to use tiles as images from the server. Therefore it only works when you are connected to the Internet, no download possible.
The tile server I'm using is free but forbids bulk downloads of large areas and demand that the tiles are cached on the Client Hard disk (for at least 7 days). Mapparium cache has no timeout currently, it just cache forever, but you can clear it using the Button in Prefs (good to define a min zoom Level which should stay. The Low zoom levels never really change.) SO in principle for offline use you can just look at the areas when you are online and the tiles get cached to the HD, these parts are available in Offline mode as well.

The AltiVec question is already answered I guess, in fact I have no knowledge about such stuff (never programmed/used PowerPC before) I'm not sure, if AltiVec would help at all, (at least on my Mac Mini) the Map feels very snappy and smooth so already "fast enough" whatever that means

Thanks for the localization offer, currently MUIMapparium has no locale.library support, but I plan to change that. I never worked with locale.library I have to learn how to do that (in FreePascal). But when included I would be happy someone try to make a translation.

About GPS: MUIMapparium supports the most common Formats for loading Tracks from your favorite GPS if not just tell me the format maybe I can include that. GPX works best and is the native format for MUIMapparium. Direct support for GPS devices is planed (until now it's a separated program, GPSTool, see earlier in the Thread).

The AREXX port is not activated currently in MUIMapparium, so your tools mentions earlier will not work with MUIMapparium currently, but I will include that again, with the same commands and footprints.

One personal side note:

For me it's very interesting how different every Amiga style OS community react on such a "new" program, I noticed already for other stuff I published and happened for Mapparium/MUIMapparium as well.
AROS: barely any downloads or feedback and if just some "thanks"
Classic: many downloads and quite a number of "thanks"
OS4: some downloads and some rather strange feedback, mainly complains about "nothing", some destructive critics
MorphOS: many Downloads and a lot of wishes/new ideas and constructive critics

All together MorphOS community: you are awesome