Transifex was considered, as the rest of MATE uses it. Welcome should be added to Transifex in future, but most likely not before 16.04. As @Wimpy is pushed for time, and found Transifex to be technical for integrating Welcome, we opted for plan A: Poedit and BitBucket. 
As members have expressed interest and may be new to translation, I wrote a guide to help. Otherwise, Welcome wouldn’t be multilingual in time for 16.04 LTS! 