I got a question today about using solutions developped on a base language (ie: English) and using them on an implementation with another base language (ie: Dutch).
This was an issue in CRM 4.0 as you had to "hack" the generated XML in an unsupported way to use a customization in another base language.
After a bit of research, this is indeed a lot better in CRM 2011 as there's no block importing a solution from one base language to another.
From this SDK article, I quote "If labels for the base language of the target system are not provided in the solution package the labels of the base language of the source are used instead. For example, if you import a solution that contains labels for English and French with English as the base language, but the target system has Japanese and French with Japanese as the base language, English labels will be used instead of Japanese labels. The base languages labels cannot ever be null or empty."; thus importing an English based solution on a Dutch based system is no issue (of course you'd better have a Dutch translation in your solution but even that is not mandatory)
The only tricky part is the line above "When a solution is imported, labels for languages that are not available in the target system are discarded and a warning is logged."
Thus if you add a MUI language after you have imported the solution, the corresponding labels will not be available and you would have to force an update of the solution or extract the language translation for a manual import.
Also, CRM2011 Rollup 1 fixes a related issue with MUI Report Wizards ("When you import a solution that contains wizard reports and the wizard reports use different languages, the solution cannot be imported if those languages are not enabled")
Thus the recommendation would be to have all needed (ie: that the customer intends to use) MUI packs installed before importing solutions.
I still need to make a test environment with an English based, Dutch and French solution (what we mostly use in Belgium) and importing in an unmatching setup (Base Dutch+MUI French or vice-versa) to see what happens when the original base language is not present as it seems to trigger some issues.