I've seen other similar questions (like this one), checked Microsoft documentation (like all the ones stemming from and related to the page Localization of Sandboxed Solutions in SharePoint 2010), plus other articles (JohnWPowell and SPbits).
Anyway, I'm not able to solve the following issue:
having a SP2010 Sandboxed solution in VS2010 with a code-only webpart, how do I localize the webpart's Title, Description (in .webpart file), Group, QuickAddGroup (elements.xml)?
I'm able to localize feature title and description, as well as webpart content by code (e.g . some label's text). I'm also aware about custom Attributes in order to localize webpart Properties when the user edits them in browser.
I was able to do this in SP2007, through some Visual Studio random magic (BuildAction + CopyToOutput + CustomTool + Deployment Location + Deployment Type + editing manifest ... don't ask me how :) ), but most of those options are not available in a Sandbox solution.
After re-reading docs, I've spotted this note which puts the final word at least on the .webpart file:
Strings in the .webpart file of a sandboxed solution can be localized only if localized resource files have been separately installed to the file system as part of a farm solution. In most situations in which you are creating a sandboxed solution, it is because you do not have permission to install farm solutions on the target farms. In such cases, there is no practical way to localize the .webpart file in a sandboxed solution. This means that the name of the Web Part in the Web Part gallery is the same in sites of all languages.
Any hint on how to do this at least programmatically, e.g. by adding the webpart to the gallery in a feature event receiver?