Skill Groups and Multiclassing

If you obtain a skill group from a class, it is considered one of your groups for the rest of your levels - regardless of what classes to take thereafter.

A common house rule is to allow skill groups to be used only for the class in which you are currently advancing, as some people consider the standard rule too open to abuse. I commonly use this house rule myself.

Skill Groups and Speak Language

Every time you spend a skill point on the Academia skill group, you gain another language.

Stacking skill ranks from different skill groups

The third paragraph under the "Skill Groups" heading on page 78 of the rulebook covers this: ranks gained from different groups stack, but only up to your level-based max ranks (level+3 for most classes, level+5 for a Thief).

So a 1st level thief with 4 ranks each in the Social and Theatrics groups would have 6 ranks in Bluff. Any other 1st level character would have 4 ranks.