Update French and Spanish translations for release

[inguin]

@Djiko @perdigano: After years of lazy idleness I have finally found the motivation to prepare a new release of my moneybalance app. The biggest new feature is the support of multiple currencies in a single calculation, which also required adding some new strings.

Could you please provide updated translations for the following strings:

• calculation_currency (changed from "Currency" to "Main Currency")
• manage_currencies
• additional_currencies
• add_currency

@perdigano: I'm awfully sorry for the long wait! I'll get a new release out ASAP.

[inguin]

Closed by pull requests #16 and #17.

[Djiko]

@inguin I'll take care of a complete review. Some strings are hardcoded in the AboutDialog activity though. Is this wanted?

[inguin]

@Djiko Great, thanks! The AboutDialog is certainly a candidate for translation, I just never got around to implement it. Also, more importantly, it should display the application version number and build date.

[Djiko]

@inguin Done in pull request #19