If I give a race the Envoy and Magical Linguist racial qualities, would the granted spell-like abilities stack to allow more uses of overlapping spell-like abilities?
Wow, these rules questions get buried fast.
They should get the full benefit of each thing, so in this case:
Envoy (1 RP): Prerequisites: None; Benefit: Members of this race with an Intelligence score of 11 or higher gain the following spell-like abilities: 1/day—comprehend languages, detect magic, detect poison, read magic. The caster level for these effects is equal to the user's character level.
Magical Linguist (2 RP): Prerequisites: None; Benefit: Members of this race gain a +1 bonus to the DC of spells they cast that have the language-dependent descriptor or that create glyphs, symbols, or other magical writings. They also gain a +2 racial bonus on saving throws against such spells. Members of this race with a Charisma score of 11 or higher also gain the following spell-like abilities: 1/day—arcane mark, comprehend languages, message, read magic. The caster level for these spell-like abilities is equal to the user's character level.
One is based on intelligence and the other on charisma. If they have both the stats then they get to cast comprehend languages and read magic 2/day and the rest 1/day. If they had a low intelligence then they wouldn't get any benefit from Envoy. If they had a low charisma then they wouldn't get the SLA of Magical linguist.