Set language via AJAX

[techknowlogick]

Fix #10725

Edit: putting this in 1.13 milestone as gitea.com's logs are full of search engines using ?Lang=lang

[codecov-commenter]

Codecov Report

Merging #12785 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12785      +/-   ##
==========================================
- Coverage   43.26%   43.22%   -0.04%     
==========================================
  Files         650      650              
  Lines       72048    72048              
==========================================
- Hits        31172    31146      -26     
- Misses      35837    35856      +19     
- Partials     5039     5046       +7     

Impacted Files	Coverage Δ
services/pull/update.go	51.92% <0.00%> (-5.77%)	⬇️
services/pull/temp_repo.go	26.59% <0.00%> (-3.20%)	⬇️
modules/notification/ui/ui.go	80.55% <0.00%> (-2.78%)	⬇️
services/pull/check.go	47.69% <0.00%> (-2.31%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
services/pull/pull.go	41.57% <0.00%> (-1.16%)	⬇️
modules/git/repo.go	49.74% <0.00%> (-1.02%)	⬇️
models/notification.go	66.13% <0.00%> (-0.92%)	⬇️
models/error.go	34.81% <0.00%> (-0.86%)	⬇️
modules/queue/unique_queue_disk_channel.go	55.38% <0.00%> (+1.53%)	⬆️
... and 1 more

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